What does the doctrine of ‘factum valet quod fieri non debuit’ mean?

The maxim "factum valet quod fieri non debuit" means "What ought not to be done becomes valid when done": a fact cannot be altered by a hundred texts. Under classical Hindu law, especially in the Dayabhaga school, an act done in breach of a merely directory or moral rule is not invalidated once it has been done. The doctrine does not, however, cure an act that violates a mandatory rule or goes to the root of the transaction.