Hi,
I just noticed that I missed a message as well yesterday, coming from an android user. The user said one tick was shown, i.e. not delivered to the mobile (I think). I didn't get any notification. Here are some excerpts from the log, it seems to have something to do with encryption:

<message from="xxx@s.whatsapp.net" id="XXXXXXXXXXXXXXXXX" notify="Someone" t="1452872879" type="text">
<enc type="pkmsg" v="1">
(some encrypted characters following)
Encoding message from xxx@s.whatsapp.net
OUTGOING:
<receipt id="XXXXXXXXXXXXXXXXXXXX" to="xxx@s.whatsapp.net" type="error">
<error type="plaintext-only">
</error>
</receipt>

I did some tests yesterday with another phone running android whatsapp, normally the message seems to be re-sent in plaintext by the other phone when my mobile sends this plaintext-only error message, but in this case it didn't.
I don't think my phone was offline at that time, there are no notifications that it went only shortly before getting that message.

If more information or more excerpts from the logfile are needed, please PM me.

19.1.2015: Correction: The other user uses an iPhone, no problems with Android users yet