I recently started playing chess online. Had two back to back matches where I was trailing behind quite heavily in terms of score (-8, -9), but managed to win!.
In Game 1 I offered a piece which opened my file directly to White King and the other guy took it. Since this was blitz and hence huge time pressure, I was counting on that mistake to happen!
I was even more pleased with myself in this game (and other guy should be equally displeased) since here also, I offered a rook on a platter and the other guy took it. Originally knight was on E7, I moved Rook to E7 and then black decided to kill it with its Queen. I moved my Queen from D4 to G7 and it was game over!
Always assuming that other guy made mistake is somewhat costly. Happy Checkmating!