Must be just me, since I can’t imagine nobody else would have found this out by now otherwise, but . . .
When I click on a banner ad (whether a normal left-button-click or a right-button-“open in new window”-click), all I get is an error message saying “The document contained no data. Try again later, or contact the server’s administrator.” This isn’t ad-dependent; it happens on all of the ads I’ve clicked over the last couple of months.
Netscape 4.7x, Win98SE, in case it matters.