I was recently at one of those science shows where, as part of the demos, a couple of volunteers were charged up with a van de Graaf generator so that their hair would stand on end. Standard stuff, except for a comment by one of the presenters: "If you're wearing metal, like a zipper, don't volunteer." The stated reason was that it was dangerous as the metal would store the high-voltage charge enough to give you a sizable shock.
Is this so? How much static charge will a zipper store? Why did the demonstrators warn us about this?