One of the problems that we are seeing pop up internally is with groups that for whatever reason have not upgraded their clients to Orcas, but are starting builds with VS 2005 clients on an Orcas server. This scenario is completely supported, but they seem to be running into one main problem.
TF215036: The build request cannot be queued at or below position 1 on agent \tp\agent1. The resulting position would be 2.
That error message is given to a V1 client trying to start a build when there is already a build in the Queue. In V1 we didn't have a queue, so for V1 to work correctly, when we get a Start Build call from the V1 web service we throw an error unless the build can be started right away. But why is there one in the Queue? What seems to typically happen is that at some point the Build Agent is rebooted or shut down for some period of time. During the shutdown, a build is started. Since the agent is unreachable by the server, the server errors out the build and marks the agent as unreachable. The next build that is started is Queued, but never actually runs, because the server doesn't retry starting a build on an agent that is unreachable. Now, the V1 client is stuck in limbo. There is no way to see the Queue or cancel that build and starting another build just errors out.
The easiest solution is to connect to the server with an Orcas client, open up the Manage Build Agents dialog, open the Agent in question and set the status to enabled. This should cause the Queued build to start right away. Once the Queue is cleared, starting builds should work as expected.