Last month, during the tragic Notre-Dame fire, Ubisoft gave away copies of Assassin’s Creed Unity for free on Uplay, as we reported. One of the unforeseen consequences of this giveaway was that players who already owned the game on Steam decided to leave positive reviews for the game, not just in memory of the fire, but also as a show of gratitude toward Ubisoft for giving the game away to other players. The higher visibility also caused some players to buy the game on Steam. And so the world's first "positive review bombing" occurred, which Valve took the time to properly evaluate in their latest blog post.
Valve goes into detail about the unusual event of a positive review bomb, with the spike of activity and players that it included, and they had to debate whether or not to consider the reviews off-topic since they primarily referred to the Notre-Dame fire and the giveaway, rather than the contents of the game itself. In terms of data, it didn't quite fit the pattern, as the people who left the reviews and bought the game were also playing the game, which rarely happens with negative review bombs (though it does happen on some occasions).