I wish I had checked the art assets I was going to use before the jam started. After the theme popped out I spent the whole night writing a script. Finished it. Then after a short break, I went to start mapping - and learned none of my assets were the right size/style for the engine I was using.
It took me until Sunday night to figure everything out and get them resized and re-formatted (and they look clunky in the end). But I sat on my hands for almost a week waiting to see what the theme was - when I could have been formatting those assets if I'd done a little pre-check. =/