Aggro-d Iron golems can appear to hit a player/entity even if there are blocks in between. It does not matter if it is a full block or if the iron golem can see you through them.

Ways to recreate:

Summon an Iron golem (preferably in enclosed space so it doesnt just walk around wall.)

Hit the golem.

Walk behind wall/blocks separating you two.

Proceed to take damage.