Coyote Blog is all over it:
Climate scientist Michael Mann (famous for the hockey stick chart) once made the statement that the 1990's were the warmest decade in a millennia and that "there is a 95 to 99% certainty that 1998 was the hottest year in the last one thousand years." (By the way, Mann now denies he ever made this claim, though you can watch him say these exact words in the CBC documentary Global Warming: Doomsday Called Off).
Well, it turns out, according to the NASA GISS database, that 1998 was not even the hottest year of the last century. This is because many temperatures from recent decades that appeared to show substantial warming have been revised downwards. Here is how that happened (if you want to skip the story, make sure to look at the numbers at the bottom).