Steps to reproduce:
In a single-player world, build a setup like shown in the picture
Get a horse with a jumping strength of about 0.737282 (as I wasn't able to reproduce with horses other than this one)
Go to the corner of that setup you built
Do a single very small jump while holding forward
If done correctly, your character's model will appear way above the horse and its model "freaks out" to say the least.
This report is invalid as your game was modified when experiencing this issue.