Some antipattern highlights from my personal Hall of Shame:
Yelp: "Followed a link via your mobile device? Have a giant content-obscuring plea to download our invasive native client! If you addicentally decline we'll helpfully redirect you to the site's root instead of the page you wanted. We helped!"
Adobe: "Forced to download the Flash Player installer? We'll just opt you in to installing McAfee's panic-inducing windmill-tilting antivirus soft. Natural pairing, like Jell-O and Cheez-Its!"