it's happened before, a few times
but without as much Outrage and Media coverage. Seems most every major "gun owner list" somehow manages to "accidentally" get released. Concealed carry license holders, registered "assault rifles", find a state/county/country/city that's opposed to such a thing, and you can guarantee there's been a breach.
In places where there's no law mandating reporting of such breaches, you have no idea how often or to whom the leaks occurred.
Once is an accident, twice is a coincidence, third time, well, you know the rest.