I can only think it was an oversight when the database in the backup environment was created, but while the default data file name is DAT_0001 and the default backup medium name is INSTANCE_COM, you can in fact call the data file INSTANCE_COM and SapDB will not complain.

Luckily for us it was the mirror backup that had this issue and not the production database, because when you try to back up or recover it will try to write file over the other, mess things up badly and then complain about an 'io error for header page'. Not particularly useful and nothing we found only matched our symptoms, but then I would be hoping that others were managing to avoid the same error!

Don't try this at home.