I personally know of that happening twice. In both cases it was used to patch a bug in the super old Qt version that was being shipped.

I'm sure it doesn't happen often, but IMHO that doesn't really matter. It's preserving the freedom to do it that is important. I rarely use my right to free speech (that is, saying something that somebody in power would find offensive and want to jail me for), but it's a right I would fight to the bitter end to preserve.