gotcha

gotcha n.  A {misfeature} of a system, especially a
   programming language or environment, that tends to breed bugs 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.