Overthrown vs lose possession

[PSUchem]

Surprisingly, we had an interesting one this weekend that fits in this thread. It wasn't my game and I didn't see the situation, but the jist was this:

Throw made to 3B for an attempted tag out. Ball gets past F5, and another fielder (F1? F7?), who is backing up the play fields the errant throw. While in the process of transferring the ball into the throwing hand, the ball slips from her throwing hand on the "backswing" of her throw and sails behind her over the fence.

Do you have a thrown ball out of play, or a loss of possession?