I’ve had a few breaking changes in 10 years of dailying Arch across multiple devices.
Most egregiously one time a PAM update included a new PAM config… which got applied as .pacnew, but the new PAM config was critical and I could not login with a cryptic error message.
That probably took me a solid hour to figure out, because config file conflicts is probably pacman’s weakest point. At least apt starts conflict resolution by default.