-There are 6 mob head types in the game. If we try to dispense any of those on the ground (not on mob/player that is able to wear it) from dispenser it will not work (intended behavior). However, if you try to dispense a wither skeleton skull it will drop on the ground just fine, unlike any other mob head (see screenshot 1 (before activating dispenser) and 2 (after activating dispenser)).
Explanation of the glitch:
1) Place a dispenser with wither skeleton skull in it
2) Activate it
What should happen:
Just like any other mob head, it should not dispense
Unlike any other mob head, it can be dispensed on the ground (not on a mob (for example, villager) or player).
See screenshot 1 and 2.