flask server with ⌃-Z (CRTL-Z), instead of the correct ⌃-C (CRTL-C), results in the server still bound to port 5000:
OSError: [Errno 48] Address already in use
Problem: After Flask crash, the port can still be occupied.
Check who is using port 5000
sudo lsof -i:5000
kill $PID kill -9 $PID
 + killed flask run