I've seen a handful of people talking about getting the 100% achievement prior to completing all of the Cotton Alley levels. I finished the Light World of Cotton Alley and I'm about halfway through the Dark World.
From what I've read, it looks like people were getting their 100% about halfway through the Dark World. I'm looking forward to rocking all of the Dark World levels, but just curious if anyone can shed some light on this possible glitch? Or am I mistaken altogether? : )
p.s. This game is unreal. I recall being slightly depressed when N+ came to its conclusion. SMB will certainly be no exception.