I've only seen this happen 3-4 times in the past 5 years or so, but I can see the email arrive via our proxy logs, yet Exchange (2010) can't seem to handle it so doesn't deliver it. When we get forwarded or faxed a copy of the email in question, the date in the body of the email is in a different language, ie: French, or German. 

We communicate with this particular overseas client very regularly so again this is a very rare occurrence, but I'd love to know definitively how\where the conversion fails in these instances. My google-fu is proving weak on this one.