Captured A/C Shootdown?

Does anyone know of any captured aircraft from any war that while being test
flown by the capturing force was attacked and shotdown its previous owners
or
allies?

I have just heard of a story about a P-47 pilot named Fred Hillis who is
supposedly credited with 4 German built aircraft and one German flown American
aircraft, another P-47. The enemy flown P-47 was unmarked and had just shot
down an American P-47.

I had never heard of this incident before and so far Hillis is not mentioned in
Olynyk's Stars and Bars or my copy of the American Fighter Aces Album from
1979.

I suppose it could be another example of fratricide?

John Dupre'