Character becomes unuseable when affected with partial block glitch

Put the summary of the bug you're having here

What I expected to happen was...:
When walking over soul sand in a specific orientation, I expected to partially glitch through the adjacent block

What actually happened was...:
My character started twitching and could not move. When taking a hit, the screen would twitch more aggressively. Dying did not resolve nor did removing blocks underneath the character.

Steps to Reproduce:
1. Done on SMP, walk onto a block of soul sand so that you are still "falling" when you touch the adjacent regular-sized block.
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