Due to the RadarURL option, all my text (including RSS) feeds were adding the RadarURL script code. RadarURL tells you how many people are on your site at any one time, and gives you various traffic details. Evidentally, it's on by default on Boxes 5, 11, 12, 13, 14, 15, and off by default on Boxes 4, 9, 16, and 17+.

You can turn it on and off on your account by: Log In -> Account Profile -> Turn it OFF/ON.

See this discussion for more info.