I'm not sure if this has been discovered yet, but if you have two large stair-cases set up diagonally from each other, certain characters can slip through the gap.
I'll show you how it is set up with bad ASCII.
X = place where you slip through
O = stairs
O
OOO
OOOOO
_____XO
______OOO
______OOOOO
If you want to try it out, just set up a stage with stairs like above, and try jumping into the gap with Toon Link, Kirby, Captain Falcon, or other characters that might be able to go through.
I'll show you how it is set up with bad ASCII.
X = place where you slip through
O = stairs
O
OOO
OOOOO
_____XO
______OOO
______OOOOO
If you want to try it out, just set up a stage with stairs like above, and try jumping into the gap with Toon Link, Kirby, Captain Falcon, or other characters that might be able to go through.