Well, if you rehearse a trick properly, this will happen rarely. But if it DOES happen, you could say something like this:

1. It is the part of the show......the part of the show that hasn't been rehearsed.

2. That's the first time it happened......again.

3. That's magic, it is unreliable, sometimes it fails...

4. That would have been cool if it worked, huh?

5. (For kids...) It didn't work because I forgot to say the magic words! Can you say the magic words with me? (Try the trick again...)