I wasn't sure if this should be in the OW forum or the *A forum.

I recently learned that I could buy a ticket on AA 001 stock to fly on AA metal from the US to Europe, and then transfer to a segment on the same AA ticket on Star Alliance member Croatia Airlines to fly into Croatia.

Is it unusual to be able to buy a AA or UA ticket and then fly on a partner airline in the opposite alliance? Are there other examples of this occurring with other OW and *A partner airlines?