I would say that you didn't specifically fail to do anything as it was a learning experience of why it's valuable to take a bit of your time to make sure your TOS is in clear view at all time when relevant.
The whole reason anyone makes a TOS is for you safety and sanity. Having it visible and making sure people agree to it beforehand makes sure that they are responsible for their actions from that point forward that conflict with what was already agreed to. You have a TOS, it's easily accessible, but the saying goes, "An ounce of prevention is worth more than a pound of a cure." It's one of those customer service quirks because there's always going to be someone that misses something in the TOS that will make them responsible for something. Regardless of how, why, or when they miss these things all you can do is try to be as clear and diligent as possible.
A good way to think of it is that rather than failing to do something or you being at fault is that you have learned that sometimes you need to take extra steps to have your bases covered because people will avoid reading, not look, or not even bother with the TOS in most cases. Most of the time it's not really an issue as most people don't have unreasonable demands to begin with, but when dealing with the wild world of customer service you will always find a way to see or hear of something completely new.. for better or worse.
Also, the editing fees aren't really a big deal in my opinion. I usually will give tips if I felt like I bothered an artist more than usual and they didn't charge for edits. As long as you maintain a calm demeanor and try to reason with your clients you'll maintain a good standing with most folks. Professionalism goes a long way to preventing drama or misunderstandings. It's fine to be nice or personable, but sometimes a healthy business attitude will serve you well in the long run.
Hope my insight helped you!