Without putting much of an effort, I've gotten myself stuck in these 2 spots (both screenshots are from Snowdin - Gold Grind location).

The next 2 screenshots are from the Waterfall Cove FFA map. Both of them (screenshots) contain tiles, that aren't working properly - they don't have collisions they should have, so you can easily walk through them, though I believe the intention wasn't for these tiles to be this way. (Also, projectiles penetrate almost all of the stones, but not all of them, it's also a bit weird, and maybe should be fixed as well.)

The next screenshot is from the Snowdin Tundra FFA map. I guess that this tile was an intention at first, but at least in my opinion, it doesn't look quite natural.

Also location of this on the map.

And, that's all for now. I can't say that I've found that much, but it didn't take me a long time to discover these 3 little bugs, I've found all of them within 30 minutes, and I think that even things like these are inappropriate. I hope that you'll think about fixing them, because even if they aren't game-breaking, they shouldn't have a place inside this game, for the sake of better game experience.