I inserted a field, which was named A, and then decided to change it, so I did Undo Insert Field. The field was removed, but the adjacent field had the name A, instead of what it had been.
I was able to confirm this problem. The problem is that the field name in the property inspector panel (“A”) was being edited. If you do anything to end this edit (press Return, click on the data sheet), the problem doesn’t happen. The Undo command needs to end the editing itself before actually doing the edit.
In the meantime, the good news is that there is no further damage to the database other than the changed field name. The rest of the database structure is fine.