So I was doing the quest where you get the recommendation from the Skingrad Mages Guild, and it all went fine until after I finished it. When I went to the Arcane University, I noticed that Erthor was following me. I didn't think much of it until I went to get my Mage's Staff and he was following me everywhere and actually helping me kill people! Has this happened to anyone else?

I't's not common, but it can happen. Here is some info on it quoted from UESP wiki:

After arriving back at the Guild Hall, Erthor may not stop following the player even after Adrienne has been spoken to.
Reloading the last autosave should trigger the normal events.
On PC, you may enter CompleteQuest MG01Destruct into the Command Console to complete the quest and stop Erthor's pursuit.
This bug is fixed by the Unofficial Oblivion Patch.


If you are on the PC, I highly recomment the Unofficial Oblivion Patch. It fixes this and a couple thousand other tiny things.

I haven't had any problems with Erthor, although the 1st time I met him I accidently stole in front of him and had to spend the next 1 to 2 hours finding Ingredients.