Oracle SQL Developer treats my comment as a command
<p>I've created some very basic SQL script:</p>

<pre><code>/*
  Change date format
*/
alter session set nls_date_format='DD/MM/YYYY';

/*
  Drop old tables (if exists)
*/
DROP TABLE Students;

/*
  Create new tables
*/
CREATE TABLE Students
  (
    Id NUMBER(6) PRIMARY KEY,
    LastName VARCHAR2(20),
    FirstName VARCHAR2(10),
    Address VARCHAR2(10),
    BirthDay DATE,
    GroupId NUMBER(3)
  );

/*
  Check the table was created successfully
*/
DESC Students;

/*
  Insert a new record to the table
*/
INSERT INTO Students VALUES(101, 'Solin', 'Dan', 'Beer-Sheva', '01/02/1985', 11);

/*
  Check the record was inserted succesfully
*/
SELECT * FROM Students;

/*
  Add 'AvgMark' field to the table with default of 0
*/
ALTER TABLE Students ADD AvgMark NUMBER(5,2) DEFAULT 0;

/*
  Check that the new field was added
*/
DESC Students; 

/*
  Insert 3 new records to the table
*/
INSERT INTO Students VALUES(102, 'Tal', 'Ruti', 'Tel-Aviv', '10/07/1988', 12, 70);
INSERT INTO Students VALUES(103, 'Kohen', 'Yossi', 'Dimona', '01/08/1987', 11, 80);
INSERT INTO Students VALUES(104, 'Toys', 'Vered', 'Tel-Aviv', '15/09/1988', 12, 90);

/*
  Check the records were inserted succesfully
*/
SELECT * FROM Students;

/*
  Change 'Address' data type to VARCHAR2(15)
*/
ALTER TABLE Students MODIFY Address VARCHAR2(15);

/*
  Check that the data type has changed
*/
DESC Students; -- Some comment
</code></pre>

<p>At the end of the <code>Script Output</code> i get this Error Message:
<code>ERROR: object COMMENT does not exist</code></p>

<p>If i'll change the comment to <code>Hello World</code> i'll get the same error for <code>WORLD</code> "object"...</p>

<p>I changed the whole script to avoid single line comments, and the result is the same.</p>

<p>I could avoid it again in this example but i really want to understand what's causing this weird problem...</p>

<hr>

<p>Another thing which might help solving the issue, is another weird error i have in the same script, few lines above the comment. in this line: 
<code>ALTER TABLE Students MODIFY Address VARCHAR2(15);</code></p>

<p>SQL Developer shows me an error under the last two characters (the right parenthesis and the semicolon) saying <code>Syntax error. Partially recognized rules (Railroad diagrams): ...</code></p>

<p>If i'll run the statement it'll work just fine...</p>