When the Deep Impact probe hit Comet 9P/Tempel, there was almost no change in brightness. … This outburst by Comet Holmes is extreme!

On the unprecedented brightening of Comet Holmes, as quoted in "Mystery Comet Explodes into Brightness" (26 October 2007) at Physorg.com. Within the space of a few hours between 23-24 October 2007 it brightened from a very dim and obscure object requiring powerful telescopes to see into one easily visible to the naked eye.