EDIT: This seems to be fixed as of 16w32b.
The game won't crash when the pumpkin/witherskull is dispensed on a structure that is used to create an iron golem/snow golem/wither. If this is not the case, the game will crash.
How to reproduce:
1. Put a dispenser down
2. Put a pumpkin/witherskull in
3. Activate the dispenser