CraftMetaSkull#applyToItem invokes the TileEntitySkull.b method to do profile filling.

 

It then waits until the Future returned is fulfilled.

 

This results in a deadlock if executed on Main Thread as the TileEntitySkull.b() dispatches the lookup Async, then tries to return to main using the processQueue.

 

The processQueue will never be processed because main is being held up by applyToItem, and the server crashes.