For some (many?) the key to BDSM is loss of control. Predictability is also control, in a practical sense. Personally, the greatest sessions I've had, and the most memorable moments of those sessions, have been when I was caught totally by surprise.

I remember...

Finally booking a non wrestling/beatdown session, only to get gut-punched [HARD!!] in the first five minutes. She (huff) wasn't...supposed to be that...(huff)...strong...!
Her walking up to me, standing and staring at me in my personal space, and suddenly licking my face.
Getting choked to the point of light-headedness in a strip club, by one of the most petitie women I've ever met
Having the hair on the back of my neck stand up, and turning around to see HER standing behind me in public, in full goth gear.
Specifically mentioning my past discomfort with anal play, but then she [gently but] insistently violated me, telling me I was going to learn...
Meeting a former Pandora's domme in a strip club, and having a two-hour session involving beatdown, cigarette burn, scissors, choking, and body worship, in the back room
When my girlfriend introduced me to prostate massage, got me to orgasm into her mouth, and then forcefully french-kissed it into my mouth. Then sat on my face "to shut you up"
When SHE left me with HER. And I begged her not to leave, but she did anyway. And she violated me while we were alone.
When I took two strippers to the back room, and one climbed up to my face and pulled her panties to the side. I declined to lick - so she rubbed herself on my forehead!
When I was blindfolded and having a presession discussion with the domme's assistant...only to find out an hour later...that WAS the domme, using a fake accent.
When I took two strippers to the back room, and one forced me to orgasm while she ordered the demure one to smother me to keep me quiet. And then she demanded a tip, and told me "Clean up, walk out of here, and don't look at anyone on the way out. I'll see you in a couple of months when you get over this and ask me to do it again."
When one domme was pegging me, and the other forced me to enhale weed and relax into it.
When she playfully beat me into submission with a belt, while babysitting me when my mom and her grandmother were out. [epilogue 1] I asked to play that game again the next time I visited. [epilogue 2] me today...LOL!
When my gf squirted in my face and then ground it onto me with her crotch

What were some of your most memorable surprises?