See Go Back for explanation.
BobWhoosta: This might be premature, but I would like to nominate move 8 as the first "Go Back".
Reasons: White has not settled on the top, and is attacking a strong black formation. The problem is there is no adequate way to seal the black corner in, so white is effectively making two groups. Black's move 9 then becomes a pincer stone that removes any possibility for a white base while extending from the hoshi stone at 3. White imo is now in trouble.