Comment by LanceH

A long time ago I had the Fritz engine from chessbase. It had a sparring feature where if you maintained good play it would give up a tactical puzzle in the middle of the game. It could either warn you or not. If you didn't play solidly enough, you would just lose.

As far as I can tell, they got rid of this feature. It was the only computer opponent that felt real. Like it made a human mistake when put under pressure, rather than just playing like a computer and randomly deciding to play stupid.