If you're performing at a party or with a group of friends the trick is a good laugh. Don't worry about the person saying it's not their bra. Everyone should know it's a trick. What makes the trick so good isn't the fact that you're taking her/his bra off it's the expression on that persons face when you do it. They always have to double check to make sure they still have theirs on.
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Just make sure you're with a group that has a good sense of humor. This is one of those tricks that you have to exercise the ability to judge a group or and individual.

It's a given. I think the audience already knows this. And your spec is not going to say anything other than laugh. I haven't actually seen anyone get embarrased from this effect, but always gets a great laugh when I see it performed.

The magic is that there is a bra tied to the hanks when there wasn't before.

Surely this falls along a similar effect of Scarne's (and others') "Shirt Off" where you unbutton a couple buttons of your spec's shirt, and pull the shirt off him by yanking on the collar.

Very popular effect in it's time.

And thinking about this effect, for the right audience, why couldn't you do this with with your female assistant? Or "spec".

And you know, every time I see a woman with a pink thong riding up her rear, I think of performing 20th Cenury Panties. Surely someone who loves the attention of wearing one in public, would have no ill thoughts when you magically remove it.