Well, I was messin' with my Yoshi in the 8th mission of Ricco Harbor, and I got a to a VERY high height thanks to Yoshi's spin plus then flutter-jump. When I landed, I had somehow gotten out of bounds into the background.

Since the rest of the island has no collision, I landed inside of it, which turned out to be a very f***ed up place (pardon my language, guys).

Water is only actually interactable in certain parts, and jumping gets acts very weird, as if the game can't tell where the ground is. Eventually, I got to a place where water actually worked, so my Yoshi died. I was able to make it to the area where those things I had witnessed behinf the boundries are. You can't do anything to them, or vise versa.

Doesn't make it any less odd since it's still the only area where they ever appear in the whole game.

Also, I jumped inside the blocked-off tunnel that leads to the Blooper Surfing area is the second mission. You can't get out, however, as the collison holds you inside. Speaking of collision, the tunnel's collision is actually a lot longer than the model itself is, so you're pretty much walking on nothing.

Doing some more tests as I type, so I'll report if I find anything more.