PostgreSQL asynchronous replication (and possible data loss)
When the master comes back up, any left-over WAL must be synced to the slave, otherwise the slave won't be able to carry on as a replica. At that point the slave will replay the WAL and be caught up with the master. No data is lost so long as the master comes back up. You only lose not-yet-replicated data if you lose the master.

This is guaranteed by the timeline; the slaves simply refuse to replay WAL archives except in the correct and unbroken timeline order. So long as you're correctly monitoring your slave nodes you'll see them stop advancing and know things are broken. This shouldn't happen if WAL archiving is set up properly, though.

Consider using async streaming replication as well as WAL shipping, that helps reduce the potential loss window if you do lose the master.