Thinking about all the things wrong with my assignment that I have already sent. Bloody hell.
I always do that with the papers I write, once I've submitted them. It's awful. 
I write a lot of technical analysis papers, typically 50-100 pages, and eventually I have to send them in for the safety review board. I always find stuff I could've written better or organized differently to be more concise after the fact. It still bugs me, but it really ate at me early in my career.