Brian X. Chen, for the NYT Bits blog:
After some sleuthing, Mr. Watt, who has a background in developing Web advertising tools, realized that the quirk was not confined to his site. The hotel’s Internet service was secretly injecting lines of code into every page he visited, code that could allow it to insert ads into any Web page without the knowledge of the site visitor or the page’s creator.
Yet another reason to bring your own 3G or LTE hotspot with you when you travel.
(Via Daring Fireball.)