Remember to test your uploads (make sure they can be downloaded and played on all systems you're building for) BEFORE the time limit is up.
This is what screwed up my GMTK jam submission this year. It was only my second jam, and I don't have a Mac to test on, so I didn't know the file was incorrect. Plus my windows upload was the 64-bit version, which someone couldn't play.
And because GMTK doesn't allow for updates/fixes after the time limit, I had to just delete both versions of my game, and the game page, effectively killing my entry (and my spirit).
I'm still kind of butthurt about it, but I really only have myself to blame.