Have you ever found a bug you can’t solve? What do you do in that case

It was actually in a 3rd party image viewer sub-component of our application.

We found that there were 2-3 of the users of our application would frequently have the image viewer component throw an exception and die horribly. However, we had dozens of other users who never saw the issue despite using the application for the same task for most of the work day. Also there was one user in particular who got it a lot more frequently than the rest of them.

We tried the usual steps:

(1) Had them switch computers with another user who never had the problem to rule out the computer/configuration. - The problem followed them.

(2) Had them log into the application and work as a user that never saw the problem. - The problem STILL followed them.

(3) Had the user report which image they were viewing and set up a test harness to repeat viewing that image thousands of times in quick succession. The problem did not present itself in the harness.

(4) Had a developer sit with the users and watch them all day. They saw the errors, but didn't notice them doing anything out of the ordinary to cause them.

We struggled with this for weeks trying to figure out what the "Error Users" had in common that the other users didn't. I have no idea how, but the developer in step (4) had a eureka moment on the drive in to work one day worthy of Encyclopedia Brown.

He realized that all the "Error Users" were left handed, and confirmed this fact. Only left-handed users got the errors, never Righties. But how could being left handed cause a bug?

We had him sit down and watch the left-handers again specifically paying attention to anything they might be doing differently, and that's how we found it.

It turned out that the bug only happened if you moved the mouse to rightmost column of pixels in the image viewer while it was loading a new image (overflow error because the vendor had a 1-off calculation for mouseover event).

Apparently, while waiting for the next image to load, the users all naturally moved their hand (and thus the mouse) towards the keyboard.

The one user who happened to get the error most frequently was one of those ADD types that compulsively moved her mouse around a lot impatiently while waiting for the next page to load, thus she was moving the mouse to the right much more quickly and hitting the timing just right so she did it when the load event happened. Until we got a fix from the vendor, we told her just to let go of the mouse after clicking (next document) and not touch it until it loaded.

It was henceforth known in legend on the dev team as "The Left Handed Bug"

If you can reproduce the issue 100% of the time, set a break point on the last step (as early as possible). If you walk through the entire call stack, I'm pretty sure you're going to come up to some unexpected values somewhere, or something that should be called but isn't.

Edit:

And if you're sitting at your wit's end trying to fix the bug and posting here hoping that you'll get some shining light advice, walk away. Go clear your head and come back later (preferably tomorrow or after the weekend). There have been many a time that I've spent an entire day searching for a solution to a particular issue just to walk away, come back the next day with a clear head and find it within ten minutes.