streaming replication, "frozen snapshot backup on it" and missing relfile (postgres 9.2.3 on xfs + LVM)

Today we have seen this on our testing database instance:

ERROR: could not open file "base/16416/291498116.3" (target block 431006):
No such file or directory

That database get's created by rsyncing the LVM snapshot of the standby,
which is a readonly backup of proddb
using streaming replication.

We do not put the standby database into backup mode before rsyncing the LVM
snapshot, trusting postgres ability to recover
from crash. Now we are not sure anymore... Thoughts? It's worth noting
that we have been using this method of taking a backup
for a long long time, but we only recently (after 9.2 came out we switched
to doing a streaming replica and then to take the
"frozen snapshot" backup method from the streaming replica).

Let me know if there is more information you need, or if this is a known
problem.

Thanks,

Bene