The first spanking I got that was "witnessed" was many, many years ago.
My girl friend at the time started disciplining after we talked a bit about her belt whippings growing up. At the time I told her I she was over due for another whippin and pulled her over my lap (she was naked) and started spanking. She resisted and pulled herself off my lap. She looked at me with a menacing look and I, mistakenly, said "Oh, I guess you want to discipline me". And she said Ya and I went along. (I'd had fantasies of a woman spanking me so I had thought about it).
She got a wooden spoon from my kitchen and came back and pulled my bare but over her lap and spanked me until I was gasping. That started the whole discipline thing. After that if I displeased her too much I'd get it. She got a short handled bath brush and an old barber's razor strap somewhere to use. Bath brush for displeased. Added razor strap for really displeased. Mouth soaping (Ugggg) for swearing.
One time we were out with one of her girl friends and I did something and Diane (my girlfriend) told me "When we get home we are going to have a little talk") I knew what that meant, was likely to get the bath brush. Then her friend asks something like "Oh, what kind of a talk". Diane said something like "more than a talk he's getting spanked". I gulped hard and broke into sort of a cold sweat. I knew the friend was coming home with us. I was in a panic driving home.
When we got home I was ordered to the "discipline spot" on the floor (had to have my hands behind my head). Diane undid my belt and releaved me of my jeans and pulled my t-shirt off. I stood there in anticipation, and in a panic, as they talked a bit. Worse I was aroused too as happened always when I was disciplined.
I was lectured and pulled over Diane's lap and got the bath brush harder than she's ever given it. I was limp and sobbing when it was over but had forgotten all about her friend. I had to stand in the corner for probably a half hour sobbing. I was bruised and sore for close to a week after per usual.
Every time I saw the friend after the "spanking" she'd grin and ask me if I'd been a "good boy".
Later Diane got more of her friends involved in my discipline but that's a long story for another time. Diane and me were together for a several years. Still wish we'd not broken up. She was a gem.