I've just written a simple plugin for YOURLS and I would like to share it with you.
YOURS stands for "Your Own URL Shortener" and it's a really nice URL shortener that you can install in virtually any hosting that has PHP enabled. Pretty cool, isn't it?
I was missing a way to redirect to a custom URL (instead of give an error) when the short URL does not exist. So I made this plugin. It is really simple and you can use it in your YOURLS installation (or use as a template to make your own plugins).
You can download it from: fallback-url.zip