Archiving skipped an xlog?

Hi,
we have weird situation today.

Pg 9.1.13, on ubuntu, on AWS.

At ~ 7am one of db servers stopped responding, and got restarted using AWS
console.

After reboot everything seemed to work fine.

But later on I found that one of the segments was not archived.

Segment ...7 was written (mtime of file in pg_xlog) at 6:51 (more or less)
Segment ...8 was written at 6:57
Segment ...9 was written at 7:13

then there were more.

What's interesting - in archive_status, i see .done files for all xlogs,
except for ...8. In logs the *8 xlog is not mentioned (I was looking for
error message or something).

The xlog didn't make it to destination or archiving, but instead of being
retried after reboot - pg moved to *9, and then continued.

I manually archived *8, and everythinh was fine afterwards, but I'm puzzled
on how could it happen that an xlog gets skipped? Even in case of
force-reboot.

Regards,

depesz