Mine was boxing. I'd been going to the club for a few months - this was a university club - and they put about 10 of us together for our first sparring session. Headguards on, etc.
Literally the first 2 or 3 seconds into my first round my opponent smacked me with what it still one of the hardest hooks I've ever felt, so I backed off a tad dazed and he did it again. I got belted about 6 times with full power shots before the coach stopped the round. I don't remember it being especially painful [although I did have a bruised nose for a few days afterwards] but what I do remember is completely losing the ability to think and move propely after that first shot, and just repeatedly walking into hits.
Also, in retrospect, having done quite a lot of sparring since, the guy wasn't really sparring in the usual sense. He was trying to knock me out and I don't think I was prepared for it.
That sparring session also left me with a bit of a dislike for headguards.
By contrast, my first karate sparring was fairly full-on -- bare knuckles and lots of bruises -- but because there was no head-contact I didn't find it difficult or intimidating.
Savate sparring under assaut rules varies a lot depending who you are sparring. Some people go super-super-light, others pretty heavy [and not always the ones you'd expect]. I've had bruising that's lasted for three weeks from 'light contact' sparring.