My general one is, "What's the stupidest thing I just did?" It might be iterating over a list the wrong way, or writing some very baroque statement, or even something as small as passing an unnecessary parameter to a function ($string, $index instead of just $char, say), but it doesn't matter--I'll fix it. I also have a specific one for when I'm working on Parrot's regexes: "Will this code make people think that Ilya's code is wonderfully clear?" ;^)