Odd as it may seem, your opponent actually can checkmate you through a series of legal moves. It's helpmate of course, but it still makes it a win on time rather than a draw by insufficient material.
If your king is on a8, black king on b6, you pawn underpromoted to a knight on b8, and the black knight on c7, then it is mate.
Isn't this a draw? I was white and it says I lost on time. My opponent only has a knight and king left.
http://www.chess.com/livechess/game.html?id=759853
thanks :)