IIRC it was an old domain registration in his real name that did him in. He realized it and change it, but the history was still there. Been a while since I read about it, but that seems to stick in my mind as how he ended up being unmasked.

There were a few failures I read about. It's kinda silly to argue about which specific one did him in when they all could've. But if he had been truly anonymous - if there was no way to link any one of his actions to any of his other actions - then obviously it could not have happened.