Some time ago, I was asked to help a customer study a set of crashes that had been collected by Windows Error Reporting. (You too can sign up to obtain access to crash data for your application.) The issue itself was the 325th most common crash in the ISV crash database, so fixing it would mean a lot toward improving the overall perceived stability of Windows. Fortunately, the issue was resolved relatively easily, but that's not what made the story interesting. What I found interesting was a little puzzle that faced me when I called up their crash profile.

One of the items in the crash profile report is a histogram plotting how many crashes per day were reported over the past three months. Most crash profiles take the form of an erratic graph with random day-to-day fluctuations. Sometimes you'll see a gradual trend (for example, as more and more people upgrade to a newer version). But this one had a strong pattern:

The number of crashes per day remains high for several days, and then plummet for two days, then return to their high values, repeating on a regular cycle.

It took me a bit of thought, but soon I understood why. Perhaps you can figure it out, too. Hints after the break.

If you actually stop and count (I didn't; I just eyeballed it), there are five days with high crash frequency, followed by two days with low crash frequency.

What happens on a seven-day cycle?

The five days with high crash frequency correspond to Monday through Friday; the two days with low crash frequency correspond to Saturday and Sunday.

The program in question targets a business audience. People use the program when they're at work (during the work week), but they don't use the program when they're at home (on the weekend). A program that isn't running can't crash.