What comes to my mind is :
Captured Fw190d flown by the Russians in 1945
Mirage F1 flown by the Iraki 1991
F14 flown by the Iranians

What are some early examples of an air force facing hostile a/c
originally produced by its own country's aviation industry (whether they
were sold to a former friendly nation that then became an enemy, or were
captured a/c, or whatever)? The first case I can remember reading about
is during the Allied invasion of N. Africa, when P-36s flown by Vichy
French were encountered, but it seems there must have been previous
instances -- WWI, interwar, or earlier WWII.