Relation has X orphan backversions (0 in use) in table Y
If you get this error for a regular table, don't worry. This error message is only a warning and it is harmless:
An orphan backversion is an old record version that is just sitting in the database file, but is not associated with any other record. Such version should be overwritten and space they occupy reused with new records. This process of cleanup is usually done in two steps. If you had a system crash between the two steps, it is possible that old record version remains there without being cleaned up.
This situation is harmless as it only makes your database file slightly bigger and it is fixed by simple backup and restore.
However, if you got the error on some crucial system table, like RDB$RELATIONS, you would have a hard time because backing up, restoring and other operations on the database will fail. The only way to fix it would be to manually edit the database file or extract the records.