There was this one library we depended on, it was sort of in limbo during the Python 2 -> 3 migration. During that period is was maintained by this one person who'd just delete older versions when never ones became available. In one year I think we had three or four instances where our CI and unit tests just broke randomly one day, because the APIs had changed and the old version of the library had been yanked.

In hindsight it actually helped us, because in frustrations we ended up setting up our own Python package repo and started to pay more attention to our dependencies.