I tried to install a service pack on my clustered Exchange servers. I failed over the first node's virtual server to another node and installed the service pack. Why, when I try to fail the virtual server back, do I receive a message saying that the versions of Exchange on the nodes don't match?
The message you received is actually a safety measure that prevents you from failing over a virtual server to a node that has a mismatched version of Exchange. In this case, the trick is to run the cluster administrator tool from the updated node—use the tool to trigger the failover, and you should be in good shape.