Viacom and Google aren't exactly the best of friends, and that's because Viacom reckon Google-owned YouTube intentionally encourages copyright infringement. Problem is, in attempting to prove to the world - and, more specifically, the courts - that YouTube is a "rogue enabler of content theft", it seems Viacom may have covertly uploaded their own video, disguised to make it look like it had been pirated, so they could continue to milk exposure on the site while simultaneously decrying it.
Unfortunately, Viacom's process apparently didn't involve keeping an accurate record of which video they'd uploaded and which had been put up by third-parties. That led to embarrassing incidents where Viacom demanded clips be removed, only to request that they be reinstated at a later point.
"For years, Viacom continuously and secretly uploaded its content to YouTube, even while publicly complaining about its presence there. It hired no fewer than 18 different marketing agencies to upload its content to the site. It deliberately "roughed up" the videos to make them look stolen or leaked. It opened YouTube accounts using phony email addresses. It even sent employees to Kinko's to upload clips from computers that couldn't be traced to Viacom. And in an effort to promote its own shows, as a matter of company policy Viacom routinely left up clips from shows that had been uploaded to YouTube by ordinary users." Official YouTube blog
YouTube reckon Viacom expects content sharing sites to magically "know" which clips are legitimately uploaded and which are not, while Viacom are taking the far more direct approach of comparing YouTube to Grokster (despite having attempted to buy the video site at one point). There's an in-depth analysis of the summary judgement - and links to the raw judgements themselves - here.