I just fell through a wall in GRB. It was a one in a million thing that I've been completely unable to replicate, so I'm just going to assume it requires even stricter timing and acrobatics to do on purpose than the Minus World glitch in the original Super Mario Bros., and the fact that I did it completely by accident is just one of those things. Still, it'd be nice if anyone could look into this, if only to prove that this can actually happen and I didn't just hallucinate the whole thing.
It happens relatively early on--right around the territory of the very first shadowy thing that appears and hits you if you don't keep moving, where the wall on the right is solid but has one of those moving extruding/retracting blocks, and you're supposed to jump on it and then to the left. Something about... I don't even know what happened, maybe it was a weird combination of being hit by the shadow while trying to jump for it and weird timing with the position of the block, or maybe it was just an MMF frameskip issue (which has never happened to me before, not even in I Wanna Be the Guy, which is supposedly so notorious for that that its download comes in two alternate frameskip/slowdown versions depending on which one works better on your computer,) but for whatever reason, when I tried to jump and press against the wall directly above that block (hoping the block would extrude and I would land on it,) I just went right through. I kept walking forward until around the area of those giant square wooden blocks behind the wall, at which point I could no longer move in either direction but I started slowly ascending upward, like I had found a secret elevator. This lasted until roughly the first free open area above that wall, and once I was back in the boundaries like I was actually supposed to be, all behavior instantly returned to normal.
I tried going back and doing it again but of course now I can't.