It's possible to clip inside a 1x1 gap just long enough to jump. Doing so allows players to jump on top of 1.5 - 2 block tall walls in specific situations.

Reproduction steps:

1. Build a 3x3 wall with a hole in the middle.
2. Build a 2 block tall platform next to the hole
3. Walk into the corner of the 2 block platform and hole
4. Repeatedly spam the jump button while walking forward

A video demonstration of these steps can be found on YouTube: https://www.youtube.com/watch?v=Y8kZW77DLuc