Source : Free On-Line Dictionary of Computing

gotcha
     
         A {misfeature} of a system, especially a
        programming language or environment, that tends to breed
        {bug}s or mistakes because it both enticingly easy to invoke
        and completely unexpected and/or unreasonable in its outcome.
     
        For example, a classic gotcha in {C} is the fact that
     
        	if (a=b) {code;}
     
        is syntactically valid and sometimes even correct.  It puts
        the value of "b" into "a" and then executes "code" if "a" is
        non-zero.  What the programmer probably meant was
     
        	if (a==b) {code;}
     
        which executes "code" if "a" and "b" are equal.
     
        [{Jargon File}]
     
        (1995-04-17)