Currently, the PM button on the forums uses the currently-displayed name as the username when sending a PM, which then causes issues when one tries to actually send it (user doesn't exist error).
One of the current test cases is Oink, whose current displayed name "Shas'Oink" does not match his actual username "Oink". Clicking the PM button on a post made by him will bring up the PM send dialog, but addressed to "shas'oink".