Here's the story of the infamous 1986 FBI Miami shootout with at least three instances of lost guns, the use of backup, multiple and the other guy's weapons.
All in a single and well documented incident.
The story is about a law enforcement shootout with two armed men. It doesn't provide much insight as far as typical civilian self-defense shootings go, but it does bring up a good point. If I were an LEO, I would do my best to be as proficient as possible with my partner's weapon(s) and would make sure that I at least had the basic ability to operate all the issue weapons for the department.
For an LEO, the likelihood of using a partner's weapon, or another officer's weapon is tremendously higher than that of a citizen using a scavenged gun.