Either you used nightly, explicitly non stable, rust instead of the default stable rust; or you used dependencies that have been yanked due to security issues; or you didn't commit your lockfile and implicitly upgrades everything by having to generate a new lockfile because you used a really wide range of compatible versions.

All of these options require you to go out of your way to enable breakage.

You could also be in the super unlucky state of using something that was later proved unsound in std, which is the only case where rust will break your code on stable. (Missused unsafe in std)