Writing regex that doesn't get you beat up in the parking lot after work.
I have recently become enlightened in the ways of regex. Regex has really allowed me to do some things that would have been very difficult to do with just string manipulation. But alas, I am gaining a reputation for churning out incomprehensible code because of the regex that is involved. There is a great article over on the Show Us Your Code site that reminded me that sometimes its not cool to "do it in one line of code".