An unavoidable error would be when you have no choice but to fail, like pretend I can't play basketball, but I'm forced to play, it was by force. So that was an error, or would be if there was a game.
Unavoidable errors are basically something you might not notice when typing something or writing/doing something. One example is a grammar error with a big word, so it's less likely to see that you even made a error in the first place.
