That glitch with the Koopa walking along the bottom of the screen is actually pretty well-documented. You know how the opposite edges of the screen overlap in the old SMB games (It's most obvious in SMB3 when you walk in one direction and see the colors at the edge of the screen that's behind you appear at the edge in front of you for a fraction of a second.)? The same is true of the top and bottom of the screen.
So, the anti-gravity Koopa Troopas? Yeah, for some reason the game has him NOT fall to his doom every once in awhile. Instead, he 'lands on' that invisible Pac-Man-style 'border' between the bottom of the screen and the top of it. In other words, he's walking along the very tippy top of the screen, and his sprite shows at the bottom as a result. On some older TV's you can actually see his pixelly little feet trotting along the top of the screen. Diligent turtles, ain't they?