Microsoft used its toolbar to watch which results people actually landed on after doing a search. MS used this to refine their own search results, promoting pages that people tended to "end" their searches on.
Google "caught" MS stealing results by manually adding an unlinked page with a unique nonsense string on it into their index. Then they installed the Bing toolbar, searched for the string in Google, and clicked on the only result. Soon after, the same result showed up in Bing.
Google claimed that Bing must have "stolen" the search result because it's not linked from any other page, so there was no organic way for Bing to find it. But it seems pretty normal to me.
This assertion appears to be supported only by your use of scare-quotes around "caught." Is there any further detail you can supply as to why Google's charge should be treated with skepticism? How else would the page have gotten into Bing?
I think the question isn't how the page got into Bing, but whether that's a problem. According to sp332, Google ran their experiment with a Microsoft toolbar installed. That toolbar is reporting back to MS about what you visit, and that's how the page got into Bing. Nothing nefarious there, and not "stealing" results, but simply monitoring users.
Google never complained about the page getting into Bing. They complained that Bing indexed it with the exact (phony) search term that Google did--Google purposefully didn't include said term on the page itself.
His post explains how. Did you read it? Installing the Bing toolbar opted users into gathering data on search traffic (including where a search ended up). Bing used that data to add pages to their index.
Microsoft's excuse ultimately did check out. That's still irrelevant with respect to my original point: it was a PR loss for them both because it revealed the ugly mudslinging sides of both companies and detracted from their images.
Google "caught" MS stealing results by manually adding an unlinked page with a unique nonsense string on it into their index. Then they installed the Bing toolbar, searched for the string in Google, and clicked on the only result. Soon after, the same result showed up in Bing.
Google claimed that Bing must have "stolen" the search result because it's not linked from any other page, so there was no organic way for Bing to find it. But it seems pretty normal to me.