I see a lot of people asking "How could this possibly happen?" It turns out that there's a common failure mode that makes it past almost every unit test suite:

I see a lot of people asking "How could this possibly happen?" It turns out that there's a common failure mode that makes it past almost every unit test suite: a lack of disk space. Running out of disk space pops up time and again. I've often wondered why software fails so often in this situation. But it's because the basic components of the system start to break -- things like "Call with temporary file" stop working, because creating a tempfile fails. And usually you're writing that code precisely in a spot where it assumes it can't fail. This just happened on my PS5. I thought I'd play a bit of Armored Core 6 over Christmas, so I sat down and was greeted by "Can't start game." There was a pending update which couldn't be installed. So I skipped update; same error. But now the update was skipped, so it wouldn't even try to reinstall it. I ended up having to reinstall the whole game. Never seen a PS5 game completely brick itself like that. Disk space woes are also one of the primary reasons websites go down. Ever forget about a server for two years and suddenly see your service break? It's almost always because the process ran out of file handles (resource leak) or because it ran out of disk space (hello imagemagick tempfiles). Of course, the bug here could be completely different, but disk space errors are so common that it's as good a guess as any. Computers don't do well when you're out of disk space. MacOS freezes and bluescreens instead of going to sleep, because it can't save the hibernation file to disk. PS5 media recording stops working, because the hour-long recording backlog no longer has room to write the video to disk. This software update obviously passed their unit tests, but failed in this particular case; disk space is one of the few variables that they probably didn't test for. I wish it were easier to write unit tests for all possible out-of-space situations. There are just so many. Even if it technically doesn't break, it often craps up the experience so much that the user ends up wondering why the software is so bad.