We all have them. Those intriguing fantasies that would just be impossible to do. Or just too complicated to justify the ends. My fantasies don't have to comply with physics or physiology, but I do live in the very real, unforgiving world of gravity and possible nerve damage.
I am lucky to have some private partners who understand when things go wrong when I experiment, yeah, we all have to pack it in. Despite what my advertising might lead you to believe, I am not infallible and all-knowing.
Professionally, I am constantly curbing expectations and even rejecting sessions because I know they won't go well. It is important to create positive and amazing experiences. This example, from several years ago, stands out as a clear example of how I work to temper people's ideas.
He wanted an hour session of:
15 minutes in the vacbed.
Golden shower.
15 minutes of inverted suspension.
Golden shower.
15 minutes of intense verbal humiliation until tears.
Golden shower.
15 minutes of puppy play.
Golden shower finish.
Whew. Funny. I could actually do this. I am actually physically capable of creating this scene to this specific outline, but . . . Not only would there be massive prep and clean up, and the manic energy to exicute this scene would exhaust me for the rest of the day, but
I CAN'T HAVE PEE ALL OVER MY BEAUTIFUL, CLEAN DUNGEON. Pee games are for the end and right before the shower. I'd never get it out of the leather.
My selfishness

aside, there is no possible way he would enjoy this. By trying to get his "money's worth" for the hour, he would rob himself of any joy to soak in the experience. Not only that, it robs me of my craft to see what is working and adjust the session's flow to match.
We ended up not playing. I still wonder if he found someone to fulfill this dream in under 60 minutes.
Also, I often reject forced-bi scenes because when faced with the reality of it, people really freak out. I find it much safer and more fun to "train" for forced-bi and keep the fantasy going.
What fantasy do you keep in the back of your mind, but maybe the reality wouldn't be so great? It's okay to have impossible fantasies; that's what imagination is for. Conversely, are there fantasies that you thought were impossible that became your reality?