Minecraft wiki entry for Memory, associated text: Setting both inputs high simultaneously is a "forbidden" condition, generally something to avoid. In the truth table, S=1, R=1 breaks the inverse relationship between Q and Q̅. If this happens, the player will get "undefined behavior" — various designs can do different things, and especially Q and Q̅ can be high or low at the same time. If the forbidden state is co-opted to toggle the output, the circuit becomes a JK latch, described in its own section. If there is instead a third input which toggles the output, the circuit becomes an "RST latch".