When I dropped the helmet I expecting the game wouldn't crash

What actually happened was... the game crashed

1. Pick helmet (I chose diamond)
2. Drop helmet
3. Collect helmet
4. Crash game
5. Receive crash report

How to fix
Using NBT-Edit I compared my Level.dat file with a new fresh one.
In NBT-Edit the Byte "CanPickUpLoot" was 1, instead of the default 0. (In NBT-Edit: Data > Player > CanPickUpLoot)
For multiplayer use the player.dat files