My friend and I played the endless setlist for Rock Band III via the Setlist Menu and the achievement never popped for both of us. We did fail one song (and hit continue) before turning no-fail mode on, but I thought that didn't disqualify us.
I dropped the Bass once in between songs which I guess dropped me out. My friend was on drums and didn't do anything like that except change his difficulty level.
We also had guests dropping in and out as the day went by.
Is there any reason he shouldn't have gotten the achievement?
Also is there any way we can edit our save files or do we have to go through that again? Thanks.