Opcode 'xx' not supported

[christandlg]

I have observed that occasionally (depending on message length) the receiver of a message displays an 'Opcode xx not supported' error. This issue may be related to !14..

Steps to reproduce:

1. build .exe from examples and run server and client on separate machines (issue also exists when both are running on the same machine, but much less frequent)
2. send a large payload (in my case, 8 MBs of 'a')
3. observe error message: