They lose containment fairly often, at least the ones I've seen. The "filling" comes out and burns through anything between it and the core of the earth.
By the way, the Pop Tart does not have enough "filling" to reproduce this effect adequately. The new Hot Pockets brand Apple Fruit Pastries, however, do a more than adequate job of emulating the 71,000C temperature experience of a McD's 'Apple Pie'.
It's pretty good pie though.