I have.
Once, in a pool hall in college, there was a kid that was looking to start up a fight. He was at the table next to ours, and he accused us of bumping him, putting him off, or whatever, and didn't feel like "sorry man" was adequate compensation for his grievance. So he's running his mouth louder and louder, starts coming around the table with fists clenched, etc.
I looked over to a friend of mine and told him very clearly to call the station for backup (I really did want him to call the cops), which froze Captain Loudmouth dead in his tracks. "You're a COP?!?!" (I'm not) to which I responded "Wanna find out?" as I squared off to him and started walking forward slowly. He didn't advance any further, so I suggested that he pay up and leave. That was enough for him, and he gathered up his stuff and left, rather meekly I might add.
I was prepared to fight him if I had to, but as it turned out, that wasn't necessary.