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?

Because I never know what to expect in a session, I am constantly surprised by moments or activities in some way so I could probably write a book on the subject. Therefore, unlike what I normally do I will try to keep this answer brief and just pick one.

My largest surprise had to be the time where my Domme decided that we should switch for a session so she had me lay down on the medical bed with my eyes closed. Meanwhile she did some self-bondage with rope where she hung herself from one of the hooks in the ceiling. When she was finished, she told me to open my eyes and I was so stunned that I literally didn't know how to react for a full minute (she told me later she started panicking a little when I laid there in an almost uncomprehending fashion before I finally got up and came over to her).

That's a hell of a surprise!!! Honestly, it would take me a minute to figure out what to do, but it sounds like she knew you would know!

Most of my experience with surprises during sessions have been rather subtle (and some of them have been negative). However there is one that I have been think about a fair bit of late even though it occurred long ago (at least 12 years)

There was a session at a dungeon. The manager asked if it was OK if a domme who was being was included (no extra cost). I said sure. It was a very intense CP session. Essentially I told the Domme to make an all out effort to make me cry. I was foregoing a safeword. She was sure she could succeed using a cane.

Early in the session I could hear her say some informative things to the trainee who barely said a word. When the caning got underway I was blindfolded. After that I only heard a little bit of training talk. However after a while I began to hear a different woman's voice talking in my ear as she used her body to help immobilize my body for the worst of the caning. While she was doing this she frequently grabbed my genitals. The way she did it was not in a pleasure giving way (to me) but neither was it true CBT but rather as if she enjoyed grabbing guys there in an assertive manner.

I was increasingly suspicious that it wasn't the trainee I was hearing. And when it was all over and the blindfold was removed there were two women in the room but neither of them was the trainee. The second woman was in street clothes! I didn't know what to make of this. I thought the dungeon might be trying to charge me for a double Domme session I had not requested. But no there was no attempt to charge me for the second lady. A cane had broke during session and I offered to pay for a fair price for that.

I asked the mystery a few questions and found out she was an independent. At the time I was very tempted to ask if she had a business card but didn't (thinking back I am not sure why. I guess I was worried the main Domme might feel slighted if I did. I can be silly that way sometimes).

Oh and I did NOT cry. I did do some begging of the mostly real kind though. I was a bloody mess and my butt took a very long time healing. After this session I decided that I had better accept that pain alone would not make me cry. I also decided that heavy bleeding should be a hard limit.

Another cliffhanger, Man!

They broke a cane on you!? Holy cow! I'd never heard of the superhero called The Thomas.

He saunters into the dungeon, saying "Don't worry about me; worry about your cane." LOL!

Well, she knew I was a switch who has a decent amount of experience on the Dom side so yea... but it really did throw me because my mind wasn't right. In the end we both had a good time though... just took me a minute or so to get my bearings.

Luke... didn't you realize that Thomas is a complete hardass in session? In fact, his ass is actually made of adamantium.

Very nice Thomas... I have had several experiences where other Dommes have unexpectedly shown up but nothing like that where I literally had no idea how many women were in the room and involved.

Although I do have to say, I am such a wimp that I would have been crying as soon as I saw a cane let alone what I would have done if a Domme broke one off on my backside.