When running a Windows 2008 (R1) Terminal Services, and you connect with a RDP 7.1 client and try to shadow another session, it may happen that both clients get disconnected.
KB2523307 tells us that this is caused by RemoteFX being active when a new client is used.
A hotfix is available to address the issue, and can be requested via the KB.
EDIT: for your convenience, I've attached the hotfix (English only!), at least until Microsoft requests its removal...