Using Profiles for Data Entry
In Drupal under Configuration > Development > Logging and Errors ie admin/config/development/logging you can set Errors and warnings to be 'off' and it is normal to do so on the production site, while leaving them showing on the development version.

The reason you don't see this on 'edit' but do on 'create' is, as you say, because Edit uses the overlay, but if you were to right click on the button instead and open in a new tab then I would expect the error is showing.

This doesn't answer your 'how do we fix it' but might explain how to avoid the error showing to users.