I discovered it on this level (Game Boy Color):
https://www.lexaloffle.com/bbs/?tid=785

To reproduce:

1. Step on the display (blue voxels)
2. Go left. You should notice your feet already sinking into the floor. There's no stepping up.
3. Once you reach the grey area with text (including CE), your character should suddenly sink chest-high and be unable to move until he jumps.

Another occurence, I think it's the same bug or at least related.

Eartbound level
https://www.lexaloffle.com/bbs/?tid=749

To reproduce:

1. Kill the monsters in first room, take the key and leave.
2. In the second room there's a ladder with a picture which is meant to be hanged. Approach the ladder from the left side.
3. Step on the ladder.
4. Walk up into the wall.

You will fall INTO the ladder. Effectively through the steps as if they weren't solid. You'll be trapped beneath the ladder until you break it.

I'll use this thread as a place to post examples of collision bugs. Here is another example of a physics bug.

Play this level - Dungeon Runners (4734)
https://www.lexaloffle.com/bbs/?tid=889

To reproduce:

1. Walk to the next room (up).
2. The gate will close behind you. Hit it with your sword only once (don't jump)
3. You can walk through the gate (you might have to move one voxel to the left or right), even though the opening is only 1 pixel wide.

I've found another physics bug. After certain jumps my character is stuck and can't move.

To reproduce:

1. Play this level: https://www.lexaloffle.com/bbs/?tid=975
2. Jump on the diagonal wall in the lower left corner
3. From there, jump onto the wall. You should end up up from pillar.
4. Jump down - onto the pillar.
5. Hold down the "down" key to make sure you're as much in front as possible.
6. Finally, jump right.
7. Try "right" and "left" movement keys.

You'll be turning around in place but won't move. You can shoot, too. But you'll be stuck until you press up or down.