The bright side of this anomaly—if it has a bright side at all— is that being ill-prepared is not the worst thing that can happen in a magic performance. Admitting to one’s ill-preparedness is.
Why magicians do it, I have no idea.
But here’s a quick tip: please don’t open your show with, “Sorry, folks, I just received this trick in the mail, and I have not yet studied it well.” Or, “I still have not yet perfected this trick, so please forgive me if I mess up.”
It’s bad enough not to practice a trick to perfection. It’s badder to have the nerve to perform it before an audience. It’s baddest to preface your performance with an apology and to confess to your crime of incompetence before you commit it.
If you have the cockiness to perform a trick you have not mastered, you must have the courage not to tell the audience in advance that you are about to fail, and that they are about to watch a lousy show.