Data corruption while taking backup of database

Hi Experts,

We took back up of a database using the following command
pg_dump -a
Please note that when we took the back the database was available for
use simultaneously from various clients.

In the back up file that was generated we found some data that was
violating foreign key constraints. And there fore we were not able to
restore database back with this back up file.

What may be the possible reasons?

Also please tell me, is it safe if we allow concurrent modification of
data through DMLs and the same time take the back up using pg_dump?

Regards
Pushker

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