Being in a D/s relationship needs a little humor. Actually, it needs a lot of humor. There is something funny about not getting it right the first time. It is an experience and a memory.
When you are fantasizing and planning a scene, everything is good. There is no chance that the paddle will fly out of your hand because you just spent half an hour spanking someone's well-padded ass with your ill-padded palm. There is no chance that the lube you just bought will turn yucky after ten minutes of a handjob. There is no chance that being objectified as a cow, which should be so humiliating, can turn into a near-Halloween costume.
No. Reality does not differ from fantasy at all. Pffft.
The truth is reality is far from perfect. We all know that, but we still need to remember that. If you did not get it right the first time, try and try again. After all, where is the fun if you got everything right the first time? Leave something to experience for the next ten, twenty or thirty years.
Plan well, but laugh when things go south. Because things will go south and it is better to laugh than to cry foul and ruin the opportunity to make beautiful and lasting memories.