I have been consistently having this issue but perhaps its just a feature.
if I'm logged into Zimbra Admin Console... and I also have the web based client open for my mail, then I choose to view another users inbox via the admin console, my session to the web client gets booted with:
"Another user has logged in on this computer. You will need to login again to continue"
Most likely this is a security feature, but I just wanted to double-check.