A few weeks back, Doc posted a mysterious message to our support list, saying that blank headlines were showing up in his outline, and wanted to know if we knew what the problem was.

This is the kind of "bug report" that makes programming for users so frustrating. You hear a user is losing data, but have absolutely no idea how to reproduce the problem. It's something you've never seen the software do, can't imagine it actually doing. The description sounds like magic, but nothing in software happens by magic, all programmers will tell you. I believe it's happening but I have no idea why.

Anyway, the problem was still happening. We needed to get to the bottom of it. So I added a bit of instrumentation to the app. Every second it would look at all the headlines in the outline and count the number of empty ones, displaying the result on the screen. If it was greater than zero the message would show up in red. I wanted to be sure Doc would see it, the instant it happened.

I called him on Skype and said he should use the software as normal, and when he saw the number go above zero, to stop and think What Did I Just Do, and write me an email. This was my hope to try to get somewhere in the vicinity of reproducible.

He said it's reporting 9 empty headlines now. Okay, go through the outline, find them and fix them. Get that number down to 0 so we can start debugging.

So he did, and we talked a bit about this process while he was doing it.

Then when he was done, he was mousing around the outline and it happened. A headline disappeared.

I said loudly, now stop! Don't touch anything. Key question: What were you doing?

He said he was scrolling around the outline and scrolled back to the top and that was when he discovered the headline was empty.

Turns out it was very important just how he was scrolling around. He was using Pageup and Pagedown, two keys I never use. But for Doc they're central. And when he'd press Pageup or Pagedown, it would wipe out the bar cursor headline. Bing!

Would it do it on my machine? Drumroll please -- yes it would. So now the bug was trapped. I told Doc we had arrived at the Holy Moment in debugging -- reproducibility. I told him "reproducible" is the programmer's favorite word. If you can tell me the steps to reproduce the problem, then I can find it and fix it. Until it's reproducible all I can do is share your frustration.

There's a lot more to say about this. I asked Doc to write this up from his point of view. For me it was a breakthrough. Finally, a user who is working on the team with me, to help make the software work better.