I've noticed that when I'm running vncviewer in a WTS session using rdesktop, sometimes the clipboard will go crazy and vncviewer will start sending a constant flow of clipboard updates to Xvnc. I haven't entirely pinpointed when this happens, but it seems to be when rdesktop tries to take ownership of the clipboard. I haven't tested mstsc to see if this is a general issue whenever vncviewer is running in a WTS session.
The clipboard code has been modified several times since 2011, and this is not something we've seen since then. Closing for now.