Are you familiar with rollover ads? Those are the ones that expand from banners into bigger ads when crossed with the mouse. Many have what I consider to be a serious design glitch. You click the X to close the ad. And then as you move the cursor away from the ad, you unwittingly cross its real estate again, and reopen it. I've had this irksome experience this morning on one of my very favorite blogs.
I'm not against ads. They pay my mortgage. But these ones break a fundamental rule. If users have already signaled that they don't want to see it, their click must be respected. This should not be that hard to fix. A click on the X deactivates the rollover feature for that session. I bet Jeff Jarvis's kid could do it.