An invisible spider started attacking me. It was not invisible due to any effect as far as I could tell, there were no particles and the spider's eyes were also invisible. It could damage me and activate pressure plates, but I could not hit it. This may be due to chance, but when I made the hitboxes visible, there was no spider hitbox either. I suspect that this may be related to some nearby chunks not rendering/loading for an extended period of time-- see attached screenshot. 

Saving, quitting, and re-opening the game fixed the bug.