I was playing around with fireworks in the latest snapshot (49a) and I wanted to fight the Wither Boss with some neat explosions. When I tried to spawn it, either:

1- 3 Soulsand and 2 Heads dissapeared, leaving 1 Head and 1 Soulsand left and no boss (Head could also be turned into a Skeleton Head);

2- Nothing happened (as shown in recording)