I've fixed plenty of bugs which I could not reproduce and thus could not test, simply based on the source code and description of problem from customer. Based on the symptoms it was clear what the code must be doing, studying the code reveals it is clearly wrong, and so the fix was obvious.

So make a patch, make a new build, send to customer and get a call "yeah that fixed it, thanks!".

Sometimes the issue is I simply don't have time to reproduce it, like that time a blocking bug had to be fixed within 30 minutes, or the customer would have had to charter their own helicopter to get a few packages to an offshore oil platform, rather than piggy-back on the worker transport heli.

Other times it's some combination of the customers system it's running on and configuration of our software that I can't reproduce.

Not saying it's ideal, but it's quite possible to successfully fix issues without being able to reproduce and test.