There was a modified version of snes9x that showed every mistake that the user made while re-recording. A video for that modified emulator can be found at
http://www.youtube.com/watch?v=T2OytHzZ72Y. But I'm pretty sure you already know of this. So why am I posting this again? I thought I saw something similar for fceux a while ago and I want it. Wasn't it a lua script or something? I want to make a video with that kind of technology with a game hack for the NES. What's it's name? I have no idea, so i guess "Super Mario Bros. 3 (E) [h2].nes" will have to do. The hack contains levels loosely based of the vanilla ones, however it's a lot more challenging. I don't think it's kaizo hard, but it's pretty hard nonetheless. The video will probably be as funny as the one posted above, but I don't know how I can give the effect of the video. So where, if it exists, can I get this fceux mod?
edit: I didn't post that video myself :P