My plan was changed without authorization - ATT Claims it was in error, but cant explain who or how.

Do you have any authorized users on your account? If so, I would absolutely ask them if they'd gone into a store recently and done anything with the account. If no one has made any trips to the store, then it's a very curious situation indeed. You're absolutely right, a representative, whether phone or store, can not change your plan, or do anything to your account at all without your knowledge. Any phone or store rep should be able to see, at least, the location of the store at which these changes to your account were made. Also, I know that when I worked as a Verizon rep, anytime any representative accessed your account for any reason they were supposed to notate what they did while they were in your account, even if they didn't do anything. I'm not sure how similar ATT's system is to the one I used for Verizon, but, if they're pretty similar, then a rep should be able to see at least some identifying note indicating who accessed your account.

Yes, it was eventually resolved. They made a mess of the bill, but the math added up at the end and I did he my unlimited plans back.