Obscure software bugs always come in pairs

Well, for me anyway, and in MySource Matrix in particular. I don't know what it is, but obscure bugs get reported to me and shortly after I have a fix, I will be fixing the bug on one or two other systems. Yet this bug may have been in the software for years and may be extremely hard to replicate. This has been happening more and more; I think it has been three or four times in the past 12 months, which is just strange.

The strangest case is a bug in a trigger condition that was caused by the way PHP converts strings to numbers. I was so surprised we had never previously encountered the problem that I wrote a blog post on the issue. The night I wrote that post I also committed a fix to MySource Matrix. The very next day I was asked to diagnose a strange problem on another client's system with triggers. The fix I committed the night before also fixed this client's problem.

The latest case reared its head again just today. About a month ago we diagnosed and fixed a strange problem that was caused by IP addresses changing and logging a user out of MySource Matrix during the multiple requests made on the frontend (HMTL, CSS, JS etc). The session is destroyed if you get logged out, but the code that loaded a serialized class didn't first include the class file. We fixed that bug. The next week I diagnosed the same bug on another client's system. Last week I received a support ticket for the same issue. And just today I diagnosed the same problem on a system undergoing implementation. The strange thing is that we have always had systems where users have their IP changing and yet we've never seen this error.

I don't get this so much with PHP_CodeSniffer, although I do tend to fix bugs in CVS only to have someone report it via the PEAR bug tracker. They are not as common or obscure though. Perhaps the size of the project directly relates to the incidence of these sort of bugs.

I think everyone hears that these sort of things happen, and not just with software bugs, but it is still amazing to see how often it occurs.