I actually posted about an off-by-101/1 bug on MSO a week ago:
How come my association bonus didn't count towards the league, but someone else's did?How come my association bonus didn't count towards the league, but someone else's did?
Tim Stone speculates that:
maybe it's a race condition where sometimes the association bonus is recorded microseconds before your account creation date and that messes things up
Up to now though, the bug hasn't been answered.