I've registered to tournaments by mistake but it's just as easy to deregister. I don't get it. I have also made the mistake of registering to a tournament but things turned up out of nowhere and had to do them and completely forgot about the tournament. But that's hardly a mistake. It's personal stupidity or as people like to call it, forgetting. Hated myself as these were decent tournaments with decent prizes and a few dollars buy-in when my bankroll was in the tens of dollars. However, I had to suck it up, calm down the chimp in my brain and do something else to make me forget the thing that brought me all of it because I'd forgotten something else. So there it is, forgetting can be both fruitful and destroyful. Ok, enough butchering of English language.
A few other times I registered to tournaments with rebuys and add-ons when I couldn't afford the rebuys and add-ons and only noticed when I got from 2nd place to last because I was the only one who couldn't add-on. Furious that made me, and not only once. That's why I only play freezeouts. May the best one win.