A client is reporting strange subject rows in Mail for OSX:

I can't seem to find the cause or any pattern. I know however that it's only client side, because when checking in Outlook, or a webmail-client, the subject row is shown correct.
It's an automated mail sent from a system, so I assume it's possible it could be an encoding error, but I've never seen an encoding error show up like this, and it doesn't show on every email, just a few of them.
Sadly I can't show more details about the email, but I figured I'd try asking here anyway.
Any ideas?