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.