When I should be doing something else. Like sleeping (why was I writing an outline at three in the morning when I had work in six hours?) or studying/doing homework (I started writing the essay and ended writing a scene for my novel. This is a bad habit methinks) or working (the poetry muse loves the office I work in).

Sometimes even when I want to be doing something else. I once left a fun night of watching movies and eating junk food with friends early because I suddenly needed to finish that chapter (mostly because I suddenly knew how it ended). Then in the rewrite I scrapped the scene I'd left my friends to write. The mind boggles.