How can you get "WAL segment has already been removed" when doing synchronous replication ?!

We are seeing situation like this:
1. 9.2.4 database
2. Master settings:
           name            |    setting
---------------------------+---------------
 fsync                     | on
 synchronize_seqscans      | on
 synchronous_commit        | remote_write
 synchronous_standby_names | *
 wal_sync_method           | open_datasync
(5 rows)

Yet, every now and then we're getting:
FATAL:  requested WAL segment * has already been removed

Assuming no part of the system is issuing "set synchronous_commit
= off", how can we get in such situation?

Best regards,

depesz

--
The best thing about modern society is how easy it is to avoid contact with it.
                                                             http://depesz.com/