If you're writing both a client and server, manually testing corner cases can be anywhere from difficult to impossible. Ideally the client won't let them occur

If you're writing both a client and server, manually testing corner cases can be anywhere from difficult to impossible. Ideally the client won't let them occur. But you should still verify the server doesn't give up the goods. I guess you could "manually" test by using the command line? Sounds like a lot of work. In my experience, more work than automating it.