Google still reigns champion of the search engines, but Microsoft's Bing has been making a slow and steady dent. Nonetheless, Google engineers found some of Microsoft's results interesting and prepared a sting operation to root out the cause. The behavior was noticed with how Bing would return near identical results to Google for misspelled search keywords - a technological point of pride for the Google time. To catch Microsoft in the act, Google engineers basically crafted manual search results for some fake, made-up keywords - essentially results that shouldn't be produced by any engine. Lo and behold, Bing began producing identical results in enough cases to be statistically significant, solid evidence of blatantly copying the search engine and passing its results off as Bing's own.

EDIT: As a follow-up, interviews with Google and Microsoft provide each party's side of the story.