An invalid turn would be something like placing a move on top of an already-filled square or outside of the 9 grid spaces. It seems obvious to a human that you can’t do these things, but computer bugs have a tendency to do things you don’t expect. And yes, passing a turn doesn’t provide an advantage, but I listed it because it’s still technically cheating.

Placing moves in invalid locations, passing a turn, or making multiple moves on one turn?