The last time was 6+ months ago because I stopped using Arch. It happened from time to time on both of my machines, but it had a lot higher chance to happen on that particular pc than on my ThinkPad. I'm guessing it was more frequent because the main one was getting updated multiple times a week.

There was one good warning sign though: if I needed to -Syyu, something was most likely going to go wrong with the update.