This nicely encapsulates what I was hoping to say, but didn't take the time to write out. Thank you!

> Maybe development of a proper test involves some test infrastructure work that's a larger undertaking than you have the opportunity for at the moment.

This is something I've encountered many times.

> Maybe you have a solid manual/QA testing system behind you, allowing you at least temporary assurance that your fix is valid.

I would hope most people are doing this anyway, especially when a big production bug has been found.