I've often done it for several comics, usually because I'm trying to find a oneshot joke or I have the retarded thought of "How different does the art look between now and when this character first showed up". When Faye cut her hair that one day she also baked a pie, I went back to look at Faye's first appearance to compare. 2 hours later, I've realized I'm a maniac for sitting around in complete silence, changing the page every 10 seconds. I could have completed a karaoke video or finished a typical assignment in that time!
It still happens about once every few months though. That and getting lost on TVTrope chains.