What was the biggest blunder made by Boris Spassky in Game 6? I just wanted to inquire about the biggest mistake or blunder by Boris Spassky In game no. 6 against Bobby Fischer.
I am tempted to say "agreeing to continue the match after Fischer's protests!" which some argue was a key psychological defeat.  If you're looking for specific moves that were mistakes, there are a few candidates.  I haven't looked at all annotations, but have Kasparov's My Great Predecessors, vol 4 handy and looking over his notes, it would be either (notations and comments are his):


...d4?
"This move, conceding the important c4-square, was rightly condemned by the commentators.  20. ...Nf6 really was better...but the most accurate solution was the continuation 20. ...c4"


and


...Rb8?!
"The last critical moment."