I was playing around with a "whoami" test service on two different agents this weekend and I totally forgot that I had already published one of the services with a custom URL.
When I published the second service with the same custom URL it returned an error at the bottom but it didn't say what was wrong. It took me about 5 minutes to realize that I had already used that custom URL elsewhere.
It would be great if the returned error message was more descriptive when a duplicate custom URL is entered. What was returned said "Failed to update custom addresses Error: Internal Server Error"
Maybe it could return "URL already in use on XYZ service" or something to that effect?