using companions to train then taking my money back. ...
I'm not convinced this is a glitch. Technically you are 'friends' with this person anyway
(did them a favor, whatever) so it is not unreasonable that they would train you for free, as your friend. Bethesda just made it awkward to get your money back.
I use the glitch where my armor and weapons don't wear out so I don't have to carry hammers anymore.
Personally I think making jewelry to increase Smithing is a glitch. I'm not saying that making jewelry isn't skillful, but honestly I do know a few jewelry smiths and none of them know anything about making iron swords. Plus the idea of making those pieces of jewelry on that large forge just doesn't seem right either. So I do consider spamming Gold Necklaces to increase Smithing a glitch.
I also killed some mage guy the other day (he was in some ruin full of ghost.) Anyway when I killed him I looted his staff, but when I exited the loot body screen I saw the staff still laying on the ground so I picked it up and had two unique staffs from him.