Why on Earth can I start two flask dev servers on the same port?

One day at work I noticed a peculiar behaviour—on Windows, I was able to start two local flask web servers on the same address and port. My previous experience lead me to expect a EADDRINUSE error but that didn’t occur. The two servers were happily running, but only the first one was getting the requests. First instinct, try the same thing on Linux (Ubuntu) and sure enough, there I get the Port…