It is very important that all *.tdb files remain consistent over all write and read accesses. However, it may happen that these files do get corrupted. (A kill -9 `pidof smbd' while a write access is in progress could do the damage, as could a power interruption, etc.). In cases of trouble, a deletion of the old printing-related *.tdb files may be the only option. After that, you need to re-create all print-related setups unless you have made a backup of the *.tdb files in time.