Do you have that one thing that you have to constantly look up?
In python, to replace elements of a string, there are two operators. One is a strictly linear search, the other uses regular expressions. The regexp call to replace part of a string with another string is sub, and the string call to replace a part of a string with another string is replace.
I get these backward all the flamin' time, and it drives me insane. I use both of these every day, why the Hell can't I remember which is which?