Bad Programming Practices

Each post must contain one bad programming practice to add to the list. I'll start:

In C++, overload library functions by using #define. That way it looks like you are using a familiar library function where in actuality you are using something totally different.

>>21
it is always associative _{and usually commutative}.
+ is commutative on every ring. If you're doing calculations on non-ring environtments, you are utterly doomed.