I had this happen in HoL at one point... I ran the dungeon on heroic twice and had 0 issues on Ionar - beat him first try. Then, I went in once and I wiped like 4 times on him, and the time I beat him I wiped immediately after he died from his spiders. Weird... Haven't been back since cause I don't need anything from the dungeon so I'm not sure if it would happen again or not.