First of all: I hope this is not a duplicate and this is only decribing this for a villager zombie as it doesn't happen for a normal one!

When you have a villager zombie and it wears a block, the block seems to render too late, so you see the head of the zombie for a short moment.

Command:

/summon Zombie ~ ~ ~ {IsVillager:1,Equipment:[{},{},{},{},{id:stone}]}

I am not speaking about the nose or the upper part of the head