Biggest mistake...
I've made so many it's hard to pick out the biggest. I've made entire videos hat were mistakes...
The most recent blunder that I know of was in my tetris video. At the end I had it set up to have a "tv turn off" effect. After disabling/reenabling 500 layers trying to export the video, I inadvertantly left a layer turned off. Instead of a cool effect with the screen distorting and squishing up, all thats there is a stupid lens flare
Despite watching the finished product over and over checking for stupid errors, I still never noticed it untill it was too late.