Viacom is not going to be suing you for watching a clip on youtube. They’re going add up all the times everyone watched their copyrighted shows on youtube and use that number to calculate some insane amount to sue Google for.
Back when Google bought Youtube I said it was a stupid purchase because they were opening themselves up to a huge legal mess with all the copyrighted material that ends up on Youtube. It’s only a matter of time before a judge rules that the current method of making copyright holders find their material on Youtube and then have them request it be removed isn’t good enough.