When I was playing in creative, I noticed I was strangely bouncing off of the bottom of blocks. I played around with it, but when I accidentally moved the incident had stopped. After trying to replicate it, I found that it was possible to reproduce by teleporting yourself just above what would normally be the highest you can be under a block. That's an odd description, so here's the replication in steps, and then in a video. Note that I found this while playing normally, not just trying to mess with the game.

1: Create the underside of a ledge. This doesn't work on the side of walls.

2: Fly up to the ledge, so your head is hitting the top. Notice how when you fly away from it, you can fly back under.

3: Teleport yourself using commands to 0.000001 higher, or _.000001. Make sure you are not under a block while doing this.

4: Attempt to go under the block again - you'd expect to either go under, or just be to the side of the block, but instead you clip in a tiny bit and are bounced away very strangely, almost similar to a slime block (I was not using slime blocks)

I have a replication in the video provided, which really shouldn't be limited to 10 MB.