I was in the middle of a game when the stupid ad for Web TuneUp[0] appeared from my task bar over the screen. The linked image is exactly how it appeared with the auto-confirm box pre-checked. In a rush to dismiss the ad to get back to my match, I accidentally clicked the OK button instead of the tiny decline.
Of course, uninstalling that travesty was a nightmare.
[0]: http://www.ghacks.net/2016/01/01/avg-putting-millions-of-chr...