Comet P17 Holmes took everyone by surprise when over a matter of hours on 24th October 2007 it brightened from below 15th magnitude to magnitude 2.5 and naked eye visibility. It looked as if Perseus had gained a new star - a slightly fuzzy one. But it's not the first time this comet has pulled this trick. It was discovered on 6th November 1892 by E Holmes, who was observing the area around M31 from London. It was then naked eye visibility. A few weeks later it faded, then had a second outburst in mid January.
Holmes is a periodic comet which swings by the Sun every 6.9 years, so it's been past quite a number of times as a very dim, large telescope only visibility, object. There are a number of possible explanations for a comet suddenly brightening like this. One it that it has been struck by something, maybe a meteor. More likely is that it has built up a dark and dirty coating during its travels and internal pressure of solar heating has caused some or all of it to strip away.
One thing is certain - it's had a lot of astronomers very excited.