Yeah, I didn't like the mistake feature because it flags it as soon as you make the error. In my case, it was actually a miss-click, so whilst I was confident with the answer I wanted to put, knowing that it had effectively just confirmed that for me made it feel like I was cheating. I'd rather make the mistake with no feedback, find the duplicate and go through the process of working out what was wrong, just as I would with paper.

EDIT: just played a hard game, and genuinely made a mistake. Having it tell me immediately definitely was cheating as there was then only one other answer that square could have been.