how to reproduce
1. Hold "W" key to move forward.
2. Place a block anywhere.
3. Notice how the player stops moving.
Linked issues
Comments 4
http://pastebin.com/CzLkkGP4
(Although it said it's modded, in fact it is not. I am using Magic Launcher and I made sure that all mods were disabled)
See my comments in MC-13203
Please force a crash by pressing F3 + C for 10 seconds while ingame and attach the crash report here.