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gotcha: n.

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.