gotcha

n. A misfeature of a system, especially a programming language or environment, that tends to breed bugs or mistakes because it behaves in an unexpected way. 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.