Two mistake rule is correct.
The worst position to be in is upside down and nose low at low level, as in doing a barrel roll from low level. It's easy to get in this position if you don't pitch up enough at the start of a barrel roll. I have a sketch in my aerobatics manual of what it looks like..and it's most likely the last thing you will ever see. That's why the victory roll and "garbage roll" (on takeoff barrel roll) were prohibited to military airmen in and after WWII.