This may seem silly, but sometimes I get so addicted to Skritter, that even when I want or need to stop, it's hard. I think it's because I feel like I should review certain words just one more time, thinking stuff like, "well I haven't had THIS word for a while, maybe I should wait for it first so I don't go another long time without reviewing it."
If this sounds sane at all to anybody but me, I think there should be a feature that will give you the last few most important words you need to review before you quit. Like a "finish up" button. Just a wild idea I thought I'd throw out there.