We couldn’t shake the feeling that something was going on, and our suspicions became much stronger in late October 2010 when we noticed a significant increase in how often Google’s top search result appeared at the top of Bing’s ranking for a variety of queries. This statistical pattern was too striking to ignore. To test our hypothesis, we needed an experiment to determine whether Microsoft was really using Google’s search results in Bing’s ranking.
Using synthetic queries that returned (clearly unrelated) inserted results.
Update: Gruber is a bit kinder,
I think Bing’s click-tracker should obey robots.txt policies as well.