On the "Chesskit Battle" thread I mentioned the horror in junior days of losing to a player rated 887 in a game in which I had K+R+N+6P vs K+R+2P. (I blundered the rook attempting a fancy rook swap and then found that despite having four pawns for the exchange my position was still dead lost.) But I was only a 1035-rated junior at the time so that doesn't really count.
The worst of the worst in my case was this one; at the time Nigel was a somewhat better player than me and had beaten me the previous five times we had played.
Frame (c. 1630*) - Bonham (c. 1500*), 1995 Sandy Bay G60
PGN Viewer
I remember after this loss I just sat there numbly and did not get up for several minutes.
* This was during a period in which the Tassie ratings were not updated for years so we were probably both a fair bit better than this.