We had a "use" of the select a slug during a shooting. Sort of. I'll try to abbreviate this as much as possible.
Call was a dude in town in a trailer park area shooting wildly. This was in the middle of the city limits, with apartments, trailers, and houses all around. This trailer park went away in the last few years (long overdue). But it was located adjacent to a really weird intersection that is hard to get to.
So, several cops respond. This is Oklahoma. People shoot in town more than they should, as most "guy with a gun shooting" calls are a car backfiring or a kid with a BB gun. It was dark out, and the trailer park area was badly overgrown. A creek right behind it, and an apartment complex behind that.
The first two cops come in from different directions to try to hem the dude in if he was shooting. The officer that gets close first starts taking multiple incoming rounds that aren't missing by much. He screams on the radio for help, and everyone goes. He has an idea where it's coming from, but can't see the shooter. Basically, he's pinned down.
Two responding officers decide to approach from the left of where they think the shooter is. One has an 870, one has an AR. They also don't know where it's coming from, but just work toward the sound of gunfire (remember, very dark, lots of woods). They see muzzle flashes, and realize the guy is about 40 yards away. The female - we'll call her Mary - tells partner to wait while she selects a slug in the dark. Partner heard "yell commands to bad guy so he'll know where we are and start shooting at us". And that's what happened.
As Mary knelt to select a slug from behind cover, they start taking close misses. She finally got it done (apparently more slowly than her last time in training). While she's doing this, partner (a former army grunt/scout platoon dude with several shooting tours over in Afghanistan) just lights up the dude with the AR. 15 very fast rounds at the muzzle flashes (still no ID on the target), and hits him 4x. None fatal of course. He had set up his AR with a personal EOTech, since at the time we had no optics - irons and 65 lumen lights only.
Guy fell down, and everybody in the town was there by now. Everyone approaches, and treat nutjob for his wounds (2 in the arm, 2 in the belly - nothing important hit).
Mary was beside herself that she never fired a shot. She selected a slug like she was taught (although with some extra "I'm getting shot at for the first time in my life" fumble factor. She's been mad ever since.
Since then we have gone to the range and had everyone at the PD shoot the "Mary Drill" since so many people went around saying how much better they would have done. Shotgun cruiser safe like you carry it. Step up on the line. At the 35 yard line, seek cover and either fire what you have in it, (almost everyone carried buck at the time" or select a slug. 5 second target exposure. If you shoot the buck, and get any pellets off target, you fail. If you select a slug, and miss or either don't get your shot off, you fail. We went immediately after I explained the rules. Of 95 shooters, I believe 6 passed. It's not hard to pass if you know it's coming, but cold - "Do this, GO!" - it ain't so easy. And nobody was shooting at them.
Most people started carrying slugs as their primary in the shotgun. Works at 2, 7 , 25, 50 and 200 all the same. No selecting shit. (My minor soap box).
Back to the story, guy survives, and goes to prison after he admitted he was trying to kill the cops. Just one of those dudes - got drunk and decided to get into a gunfight with the responding officers.
If Mary hadn't had a partner with an AR that was ready to go NOW, who knows the outcome? This was a very dark gunfight in a really bad place as far as backstop goes. None of our guys hit, and only one of our guys shot (totally blew our great hit percentage before and since then). Shortly after this, we got much brighter lights, aimpoints on every single rifle, and most guys (and girls) carry slugs in their smoke poles. But very few even carry them anymore. The favor of the smoke pole went away rapidly after this shooting.