There used to be one guy at my gym who was a semi-pro mma fighter.
I was and still am a brand new white belt, he was a much more experienced white belt.
One day in open mat he does the finger-over-finger rolly thing at me and I was like alright I'm down, I had already rolled with this dude a couple times already and he was a chill dude. I only found out after he fucked me up that he had an mma fight coming up and wasn't interested in going easy on a guy like me. In fact he was interested in only one thing: practicing his armbars on me over and over and over. This wasn't something he mentioned before we started rolling but it became pretty obvious after the 3rd or 4th time we reset and he went straight for the same armbar. Normally I wouldn't have any problem with this, you wanna train one specific thing while ragdolling me? cool, i get to train defending against one specific thing while getting ragdolled by you :D but when I say this guy was taking it seriously as part of his training for his fight I mean that he would use 100% grip strength on my arms to isolate the elbow, secure it while getting in S mount and continue deathgripping my arms/shoulders when he swung he leg over and leaned back to nearly rip my arm off. When I got home and took my shirt off I saw that I was covered in tiny finger tip sized bruises all over my shoulders and arms, probably 40-50 such marks all over me, for a second I thought I contracted some kinda disease lol. Plus my biceps were useless inflamed lumps for about 5 days. He stopped coming to the gym shortly after and this was a couple months ago, I don't hold any grudge because I understand why he rolled the way he did. At the same time if I saw him again there's just no way I'd roll with him. He was way more experienced than me and whether or not he had an mma match to train for doesn't change the fact that he could've used 50% of his grip strength and leaned back with less force when he finally put the armbar on me, and he would've gotten the same number of armbars with the same benefit for his technique.
Btw my gym doesn't train mma, its just bjj so its not like he was coming out of an mma training session where he was going 100% and he just didn't do a good job chilling out for open mat.
TLDR: semi-pro mma dude with way more experience than me had a fight coming up and decided to ragdoll my brand spankin new white belt ass like it was his ADCC trials match because fucking up a defenseless whitey like me was apparently really important for his training lol