It sure happens that sertain things are not going as planned, and It still happens today. I think as long as you make it a part of your act, no one will really notice that something went "Wrong". I never screwed up in my close-up performances that bad that it was beyond repair, as far as I can recall. Of course I make mistakes, but I did create a "safety net" for myself through the years, based on experience and knowledge, which allows me to create an "Out" in almost any situation.
However, I did screw up -I mean really- in my stage Illusion show that I used to do. To give you one example: One of the illusions in the show was Steinmeyer's "Hologram" illusion. In the middle of the performance, suddenly the front door fell open because I didn't locked it properly...the whole thing was exposed. Very painful indeed. In this situation there was not much that I could do, exept for continuing the show and smile :)