I don't remember the details, but there was a game Karpov-Christiansen (or maybe Christan.-K), early 1990s, where Karpov blundered a piece in the first 8-10 moves. Others can correct my faulty memory on this one. But there have been other howlers as well. . .
Most Embarssing GM Opening Blunders?
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Joseph Henry Blackburne once opened with e4 and said "i resign" for a laugh, his opponent wasn,t laughing and held him to it.
Anand was following a prearranged draw, and thought it was theory.
A simul:
According to the "Nordlivlansische Zeitung", Capablanca had a cold.
I doubt he was GM-level at the time, but here are two by Euwe when he was 19.
It must have been a bad tournament for him.
Mar 25, 2013
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#10
Wow, that is an list of impressive blunders from World Champions of Chess! This time a game in which a World Champion is on the other side of the blunder. It is the second from Wijk aan Zee and the fourth from the Netherlands.
TBentley wrote:
A simul:
According to the "Nordlivlansische Zeitung", Capablanca had a cold.
Thx for these guys! Funny. 
I guess some might be understandable if doing a simultaneous or speed type of match.
Maybe it was also because opening theory wasn't as established or hashed out yet in those days for some GMs? But wow...poor Karpov.
Does it happen at the GM level, where a GM literally makes a catastrophic mistake within the first ....8 to 10 moves or so?
If sos, what are some examples?
And what was the reasoning behind it (fatigue...a reallllly bad day lol)?