Aha! I had a look and Heifer International is a "geo-targeted" ad being served by one of the ad agencies. Which may explain why some people see it and others don't.
For a while there was a problem with a Honda Insight ad that was "flying out" over top of stuff. The solution is:
1) don't run your mouse pointer over the ad on your way to/from the menus, or...
2) sign up for membership and have all the annoying ads removed!
(They help pay the bills.)