I actually made a citizen’s arrest years ago.
I was working as a campus security officer at the University of Utah and was technically some very low class, untrained “officer,” which meant that under the direct supervision of the campus police, (who were Utah Category 1 Peace Officers with full policing powers), we could do something or other, which was never specified to us.
Our primary job was to lock and unlock door, turn off unnecessary lights and keep out eyes open. We were not allowed to carry weapons of any sort, had no powers to detain people and basically all we did was have brown polyester uniforms with official looking badges.
However, one time, I did catch some college kids running around in a locked building. They had accessed it through some tunnels from another building which they also shouldn’t have had access to. I called dispatch who sent a police officer.
Not having any training, I had them go outside to wait, where the police came and picked them up. These smart ass, 19-year-old kids proceeded to give fake names and dates of birth, annoying the officer to the point where he decided to throw the book at them rather than just give them a warning.
As I had the kids go outside, the officer hadn’t seen them in the building, so he had me arrest the kids, which basically meant I told them they were under arrest and signed the paperwork. Apparently, had I kept them inside the building the officer could have arrested them.
I don’t know what happened after that and I never had to become a witness or anything.
In the three years I worked there while working on my degree, there were a couple of things like that that either I helped with or my colleagues did. Almost always, it was just kids being stupid and nothing really serious.