A quick one I am dealing with these days – if you switch the Public URL of your Team Foundation Server to HTTPS you might see your Build Agents losing connection with the server.
This usually happens because of a known bug in TFS an OAuth token isn’t registered so all the authentication tokens on the agents expire.
Of course YMMV, so always double check with Support before running a Stored Procedure on your production instance.
If you happen to get into this problem, you can mitigate it by reverting your HTTPS switch-on and changing the Public URL back to the HTTP version. Doing that will re-establish the connection between the server and the agents.