When I was 1750 USCF, I played a 1200- I was up a Rook and had a pawn one square away from queening which could not be stopped. My opponent desperately tried to perpetually check me, and I had 5 moves- 4 would lead to the eventual end of the checks and an easy win, one lead to a mate in one. Guess which one I picked.
I've just los to someone rated with more then 1000 point less then me.
It was a blitz and I am sure that guy played on internet before then he went and lose to some kids in a low sections, When I face him my blitz rating was 1700 and his was 600 and I lost, I igve up a winning position.... but he was much stronger then 600...
In rated OTB game (excluding blitz), I think the lowest rated I've ever lost to is about 1500-1550. That is because I started competing in chess only after like 5-7 years of occasional casual playing and studying. So that when I played my first ever rated game, my actual strenght was probably around 1750-1800 already (my lowest ever OTB rating is bit over 1700).