Note: Multiple users logged in via Fast User Switching can lead to confusing or
conflicting reports. When a second or third user logs in to a computer, there is no way
of knowing which user is the active user. Session length may not reflect actual usage,
and login and logout times overlap.
So, 3 users logged in using Fast User Switching can confuse ARD. Bad start. What I saw happen just today, though, was ARD getting so confused, that it wouldn't let me Observe or Control the client any more. Kickstarting ARD didn't help either. Did I mention this server was in a different time zone?
I ended up rebooting the server. Good thing SSH was turned on. I'm trying to reproduce this on another system. I'll update if I can make it happen.
So, avoid fast user switching over ARD.