Hello j_thomo -
Thanks for reaching out to us.
For an issue like this one, we can certainly look into it as the tags you describe are part of the editor as opposed to a template where the entire HTML has been customized.
The error message you report typically occurs when you attempt to Save a template you are customizing when you have added both of those options from the menu on the left of the template customization page and since those are internal tags that tell the template where to place the body of your description when you add your template to a listing, you cannot include both options as that would have your item description appear twice in the listing.
I took a look into your account at the custom template that is saved there and since it has been saved, it seems you were able to locate where the additional tag had been entered as I was able to Save and Preview the template in your Custom Templates area. If you were working on another template and could not save it due to this message, you will need to take a look at the template you are working on in HTML (use the Source button in the editor) and locate where you have the tag you do not need and remove it which will allow you to save the template in your account.
If you encounter further difficulty, please feel free to file a support case with the specifics of what you run into so we can take a look.