Self-mate.
What is the worst way to lose at chess?
The worst way to lose at chess? How about this one. My poor opponent was up the whole game and even had 11 point material advantage on me when I pulled a super sneaky move on the poor Dude. I actually felt sorry for him after this, hence why I have changed his name. Enjoy this and take a lesson.
"Sucker punch"mates when in dominant, winning position (especially when you see it the moment you've moved+don't think your opponent has & try not to react-eventually to no avail)- similar, miscalculation of move sequence by just ONE when both working towards a mate-these are bad-but the worst,for me, is when you've played well and accurately in a long,close,game-especially over many days-finally got the upper hand and with JUST ONE casual or tired move,throw it all away.
When you commit 2 HUGE MISTAKES at the end, and you resign. Game: https://www.chess.com/daily/game/270610074
Resign.