tar error while running basebackup

Hi!

We're using two backup strategies to get consistent backups of our
postgresql databases. First, we create a complete dump every night by
running pg_dump, zipping the file and writing this backup on tape.
Second, we create a basebackup every saturday. To create a basebackup,
we run pg_start_backup. After that, we create a tar file of the complete
database directory and stop the backup mode by running pg_stop_backup.
Of course, all archived wal logs are also copied ;)

Well, everything was fine for month. But from time to time, I get an
error when running tar:

tar: ./base/208106/209062: File shrank by 262144 bytes; padding with zeros
tar: ./base/208106/210576: file changed as we read it
tar: ./base/208106/210577: file changed as we read it
tar: ./base/208106/210431: file changed as we read it

How can this happen? I always thought, that, when in backup mode,
nothing is able to change the database - so the database files shouldn't
change. Can autovaccumdb cause the changes?
I already read something, that this kind of errors can be ignored when
creating a basebackup, but I can't believe that. Normally, the tar file
have to be worthless, when an error occurs - or do I have an error in
reasoning?

best regards,
Andre