gotcha

   A  {misfeature}  of  a  system,  especially a programming language or
   environment,  that tends to breed bugs or mistakes because it is 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.

[glossary]