[Zope] Two ip adresses in a row in Z2.log

In some very rare cases, I get strange entries in Z2.log : the first field is made of two IP adresses (comma-separated) instead of the usual alone IP address. It seems the problematic entries comes from someone accessing the site via a GPRS mobile phone. I digged quite hard in medusa's code, but did not find where this could happen. Has anyone some insight about it ? -- Damien