I have a non-reproducible bug (at least I think it's just one and I've failed to reproduce it) in my software. It causes the entire UI to disappear and the use

I have a non-reproducible bug (at least I think it's just one and I've failed to reproduce it) in my software. It causes the entire UI to disappear and the user ends up opening a new instance of the program. I only know it exists because about once every few months a user can't upgrade and when I remote in and look in the task manager, an instance of the program is running. I have an idea how to track it down and fix it but there's always something more important to be done.