diff --git a/.travis.yml b/.travis.yml
index 18f0bd5a..1680bf51 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -102,7 +102,7 @@ install:
   - conda install $COMPILERS
   - conda install libblas libopenblas # Prevent 'File exists' error
 
-  - if [[ "$TRAVIS_PYTHON_VERSION" == "3.8" ]]; then
+  - if [[ "$TRAVIS_PYTHON_VERSION" == "3.8" ]] || [[ "$PY" == "3.8" ]]; then
       conda install nlopt mpi4py scipy mpich;
       export PETSC_CONFIGURE_OPTIONS='--with-batch';
       conda install petsc4py;
diff --git a/libensemble/gen_funcs/aposmm_localopt_support.py b/libensemble/gen_funcs/aposmm_localopt_support.py
index d277550d..20734a7e 100644
--- a/libensemble/gen_funcs/aposmm_localopt_support.py
+++ b/libensemble/gen_funcs/aposmm_localopt_support.py
@@ -6,6 +6,7 @@ __all__ = ['LocalOptInterfacer', 'run_local_nlopt', 'run_local_tao',
            'run_local_dfols', 'run_local_scipy_opt', 'run_external_localopt']
 
 import psutil
+from libensemble.tools.tools import osx_set_mp_method
 import numpy as np
 from libensemble.message_numbers import STOP_TAG, EVAL_GEN_TAG  # Only used to simulate receiving from manager
 from multiprocessing import Event, Process, Queue
@@ -14,6 +15,9 @@ import libensemble.gen_funcs
 optimizer_list = ['petsc', 'nlopt', 'dfols', 'scipy', 'external']
 optimizers = libensemble.gen_funcs.rc.aposmm_optimizers
 
+# Resolves multiprocessing issues with Python 3.8+ on macOS
+osx_set_mp_method()
+
 if optimizers is None:
     from petsc4py import PETSc
     import nlopt
diff --git a/libensemble/history.py b/libensemble/history.py
index 6486bff2..826193ee 100644
--- a/libensemble/history.py
+++ b/libensemble/history.py
@@ -2,7 +2,7 @@ import numpy as np
 import time
 import logging
 
-from libensemble.tools.fields_keys import libE_fields
+from libensemble.tools.fields_keys import libE_fields, protected_libE_fields
 
 logger = logging.getLogger(__name__)
 
@@ -96,7 +96,7 @@ class History:
 
         for j, ind in enumerate(new_inds):
             for field in returned_H.dtype.names:
-
+                assert field not in protected_libE_fields, "The field '" + field + "' is protected"
                 if np.isscalar(returned_H[field][j]):
                     self.H[field][ind] = returned_H[field][j]
                 else:
@@ -175,6 +175,7 @@ class History:
             update_inds = D['sim_id']
 
         for field in D.dtype.names:
+            assert field not in protected_libE_fields, "The field '" + field + "' is protected"
             self.H[field][update_inds] = D[field]
 
         self.H['gen_time'][update_inds] = time.time()
diff --git a/libensemble/libE.py b/libensemble/libE.py
index eaa1053b..a4bf816f 100644
--- a/libensemble/libE.py
+++ b/libensemble/libE.py
@@ -16,7 +16,6 @@ is dumped to file, and MPI abort is called.
 __all__ = ['libE']
 
 import os
-import platform
 import logging
 import random
 import socket
@@ -34,7 +33,7 @@ from libensemble.comms.comms import QCommProcess, Timeout
 from libensemble.comms.logs import manager_logging_config
 from libensemble.comms.tcp_mgr import ServerQCommManager, ClientQCommManager
 from libensemble.executors.executor import Executor
-from libensemble.tools.tools import _USER_SIM_ID_WARNING
+from libensemble.tools.tools import _USER_SIM_ID_WARNING, osx_set_mp_method
 from libensemble.tools.check_inputs import check_inputs
 
 logger = logging.getLogger(__name__)
@@ -314,10 +313,8 @@ def libE_local(sim_specs, gen_specs, exit_criteria,
     #  switched to 'spawn' by default due to 'fork' potentially causing crashes.
     # These crashes haven't yet been observed with libE, but with 'spawn' runs,
     #  warnings about leaked semaphore objects are displayed instead.
-    # The next several statements enforce 'fork' on macOS (Python 3.8)
-    if platform.system() == 'Darwin':
-        from multiprocessing import set_start_method
-        set_start_method('fork', force=True)
+    # This function enforces 'fork' on macOS (Python 3.8)
+    osx_set_mp_method()
 
     # Launch worker team and set up logger
     wcomms = start_proc_team(nworkers, sim_specs, gen_specs, libE_specs)
@@ -434,6 +431,8 @@ def libE_tcp_mgr(sim_specs, gen_specs, exit_criteria,
     port = libE_specs.get('port', 0)
     authkey = libE_specs.get('authkey', libE_tcp_authkey())
 
+    osx_set_mp_method()
+
     with ServerQCommManager(port, authkey.encode('utf-8')) as manager:
 
         # Get port if needed because of auto-assignment
diff --git a/libensemble/libE_manager.py b/libensemble/libE_manager.py
index 452c3779..e594dc75 100644
--- a/libensemble/libE_manager.py
+++ b/libensemble/libE_manager.py
@@ -21,9 +21,10 @@ from libensemble.message_numbers import \
 from libensemble.comms.comms import CommFinishedException
 from libensemble.libE_worker import WorkerErrMsg
 from libensemble.tools.tools import _USER_CALC_DIR_WARNING
-from libensemble.tools.fields_keys import libE_spec_calc_dir_combined
+from libensemble.tools.fields_keys import libE_spec_calc_dir_combined, protected_libE_fields
 import cProfile
 import pstats
+import copy
 
 if tuple(np.__version__.split('.')) >= ('1', '15'):
     from numpy.lib.recfunctions import repack_fields
@@ -130,6 +131,7 @@ class Manager:
         timer = Timer()
         timer.start()
         self.date_start = timer.date_start.replace(' ', '_')
+        self.safe_mode = libE_specs.get('safe_mode', True)
         self.hist = hist
         self.libE_specs = libE_specs
         self.alloc_specs = alloc_specs
@@ -261,7 +263,7 @@ class Manager:
         self.wcomms[w-1].send(Work['tag'], Work)
         work_rows = Work['libE_info']['H_rows']
         if len(work_rows):
-            if 'repack_fields' in dir():
+            if 'repack_fields' in globals():
                 self.wcomms[w-1].send(0, repack_fields(self.hist.H[Work['H_fields']][work_rows]))
             else:
                 self.wcomms[w-1].send(0, self.hist.H[Work['H_fields']][work_rows])
@@ -428,10 +430,19 @@ class Manager:
     # --- Main loop
 
     def _alloc_work(self, H, persis_info):
-        "Calls work allocation function from alloc_specs"
+        """
+        Calls work allocation function from alloc_specs. Copies protected libE
+        fields before the alloc_f call and ensures they weren't modified
+        """
+        if self.safe_mode:
+            saveH = copy.deepcopy(H[protected_libE_fields])
+
         alloc_f = self.alloc_specs['alloc_f']
         output = alloc_f(self.W, H, self.sim_specs, self.gen_specs, self.alloc_specs, persis_info)
 
+        if self.safe_mode:
+            assert np.array_equal(saveH, H[protected_libE_fields]), "The allocation function modified protected fields"
+
         if len(output) == 2:
             output = output + ((0,))
 
diff --git a/libensemble/tools/fields_keys.py b/libensemble/tools/fields_keys.py
index 0c454970..bc0d3761 100644
--- a/libensemble/tools/fields_keys.py
+++ b/libensemble/tools/fields_keys.py
@@ -12,6 +12,13 @@ libE_fields = [('sim_id', int),        # Unique id of entry in H that was genera
                ]
 # end_libE_fields_rst_tag
 
+protected_libE_fields = ['gen_worker',
+                         'gen_time',
+                         'given',
+                         'returned',
+                         'given_time',
+                         'sim_worker']
+
 allowed_sim_spec_keys = ['sim_f',  #
                          'in',     #
                          'out',    #
diff --git a/libensemble/tools/tools.py b/libensemble/tools/tools.py
index 96cfa3bd..4cfd1fd0 100644
--- a/libensemble/tools/tools.py
+++ b/libensemble/tools/tools.py
@@ -6,6 +6,7 @@ and user functions.
 import os
 import sys
 import logging
+import platform
 import numpy as np
 import pickle
 
@@ -148,3 +149,15 @@ def add_unique_random_streams(persis_info, nstreams):
 def eprint(*args, **kwargs):
     """Prints a user message to standard error"""
     print(*args, file=sys.stderr, **kwargs)
+
+
+# ===================== OSX set multiprocessing start =======================
+# On Python 3.8 on macOS, the default start method for new processes was
+#  switched to 'spawn' by default due to 'fork' potentially causing crashes.
+# These crashes haven't yet been observed with libE, but with 'spawn' runs,
+#  warnings about leaked semaphore objects are displayed instead.
+# The next several statements enforce 'fork' on macOS (Python 3.8)
+def osx_set_mp_method():
+    if platform.system() == 'Darwin':
+        from multiprocessing import set_start_method
+        set_start_method('fork', force=True)

diff --git a/libensemble/tests/unit_tests/test_comms.py b/libensemble/tests/unit_tests/test_comms.py
index 87baf3b3..6c40afa2 100644
--- a/libensemble/tests/unit_tests/test_comms.py
+++ b/libensemble/tests/unit_tests/test_comms.py
@@ -7,12 +7,16 @@ Unit test of comms for libensemble.
 import time
 import queue
 import logging
+from libensemble.tools.tools import osx_set_mp_method
 
 import numpy as np
 import libensemble.comms.comms as comms
 import libensemble.comms.logs as commlogs
 
 
+osx_set_mp_method()
+
+
 def test_qcomm():
     "Test queue-based bidirectional communicator."
 
diff --git a/libensemble/tests/unit_tests/test_history.py b/libensemble/tests/unit_tests/test_history.py
index ac55d778..b7602811 100644
--- a/libensemble/tests/unit_tests/test_history.py
+++ b/libensemble/tests/unit_tests/test_history.py
@@ -188,6 +188,15 @@ def test_update_history_x_in():
     assert hist.index == 10
     assert hist.sim_count == 0
 
+    # Force assertion error when a libE protected field appears in gen_worker
+    H_o = np.zeros(size, dtype=gen_specs['out'] + [('given', bool)])
+    try:
+        hist.update_history_x_in(gen_worker, H_o)
+    except AssertionError:
+        assert 1, "Failed like it should have"
+    else:
+        assert 0, "Didn't fail like it should have"
+
 
 def test_update_history_x_in_sim_ids():
     hist, _, gen_specs, _, _ = setup.hist_setup2A_genout_sim_ids(7)
diff --git a/libensemble/tests/unit_tests_nompi/test_aaa_comms.py b/libensemble/tests/unit_tests_nompi/test_aaa_comms.py
index fc356267..7ce35bb5 100644
--- a/libensemble/tests/unit_tests_nompi/test_aaa_comms.py
+++ b/libensemble/tests/unit_tests_nompi/test_aaa_comms.py
@@ -10,8 +10,11 @@ since pytest slurps up everything (including all the modules) in one go.
 
 import time
 import signal
+from libensemble.tools.tools import osx_set_mp_method
 import libensemble.comms.comms as comms
 
+osx_set_mp_method()
+
 
 def test_qcomm_proc_terminate1():
     "Test that an already-done QCommProcess gracefully handles terminate()."

Investigate dependency issues for Python 3.8 on macOS Travis jobs
See: https://travis-ci.org/github/Libensemble/libensemble/builds/731087713
libensemble/tests/unit_tests/test_comms.py::test_qcomm libensemble/tests/unit_tests/test_comms.py::test_missing_handler libensemble/tests/unit_tests/test_comms.py::test_gen_comm_handler libensemble/tests/unit_tests/test_comms.py::test_sim_comm_handler libensemble/tests/unit_tests/test_comms.py::test_comm_eval libensemble/tests/unit_tests/test_comms.py::test_qcomm_threadproc libensemble/tests/unit_tests/test_comms.py::test_comm_logging libensemble/tests/unit_tests/test_history.py::test_hist_init_1A_H0 libensemble/tests/unit_tests/test_history.py::test_update_history_x_in libensemble/tests/unit_tests/test_history.py::test_update_history_x_in_sim_ids libensemble/tests/unit_tests/test_history.py::test_update_history_f libensemble/tests/unit_tests/test_history.py::test_update_history_f_vec libensemble/tests/unit_tests_nompi/test_aaa_comms.py::test_qcomm_proc_terminate1 libensemble/tests/unit_tests_nompi/test_aaa_comms.py::test_qcomm_proc_terminate2 libensemble/tests/unit_tests_nompi/test_aaa_comms.py::test_qcomm_proc_terminate3 libensemble/tests/unit_tests_nompi/test_aaa_comms.py::test_qcomm_proc_terminate4