Tribler can popup this error. This error means "The port mapping entry specified conflicts with a mapping assigned previously to another client"
So there may be other people on your network using Tribler, or your UPnP-enabled modem hands out different IP addresses to your machine and there is a forwarding rule for the Tribler port (7762) to a previously assigned IP address.
Tribler normally deletes port mappings on shutdown. We should add extra code that tries to delete old mappings
first before adding a new one.