I was a little surprised to see Yahoo! following the lead of Google, so fast, when it comes to randomizing the number of ads shown in a contextual ad.
The ads at the top of this page randomizing between showing 1, 2, 3, 4, and sometimes zero ads. Maybe they are just testing it but I have screen captures of those flavors to show you. So you know, as I write this, it is still happening.
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Posted a forum thread for discussion at DigitalPoint Forum Thread. After reporting it to JenSense, she captured some screen shots as well.