Invisible mobs with no hit boxes (Caused by using Optifine)

I don't know what causes this, but occasionally I encounter a hostile mob that is completely invisible and has no hitbox. I have even checked for hitboxes and they aren't there. The mob can still hurt you, and still makes noise. I have been able to fix this by leaving the world and coming back, and the mob will be normal. I have encountered this in both singleplayer and multiplayer.