If you have an effect that occasionally flashes or reveals, it may be worth having another trick that works with a similar effect, but different method, so you could say something like.
"well spotted, I was testing to see if you would see that. Now take a look at this, this one's real magic"
And do an effect which looks similar, but has a different method.
Maybe if the second one is impressive enough, and you don't make a big deal of being caught, they'll forget about it, and assume you meant them to see. Then move on to another trick. You don't have to hand out your props for people to play with, especially if you are in the middle of using them for an effect.
I guess if you imply that what you flashed them wasn't "proper" magic, but a test, they're less likely to want to do it themselves.
Just a thought. I'm not much of a performer myself.
Probably the "proper" answer is to practice until you never flash or drop anything, but as Michael Ammar says "Gravity happens". It certainly happens to me a lot
