She dissed him on Letterman.
He hit back on a celeb news site.
But it turns out that the latest Hollywood feud - the actress Mary-Kate Olsen vs. the celebrity gossip king Spencer Pratt - actually goes way back!
Seeds of hate were planted years before Mary-Kate Olsen told David Letterman about her high school classmate Spencer Pratt's ill-tempered ways.
In a 2007 Details article about Brody Jenner entitled "Screwing Himself to Stardom," his best friend at the time, Spencer, bragged to the magazine's reporter about how he once made $50,000 by selling a photo of Mary-Kate Olsen.
The shot was taken of Mary-Kate, underage at the time, drinking at a party.Mary-Kate, who along with Spencer and many other celebs, graduated from the Crossroads School in Santa Monica. She has since become protective about her privacy, often disallowing guests in her homes if they have cameras.
[NOTE: Spencer's scheming did not end there. Pratt talks openly to Details about include having Brody Jenner start dating Nicole Richie merely to try to get her to eat (he failed), then date every girl on MTV's The Hills.]
Perhaps most amazingly, Spencer once talked about how he wanted to make a sex tape with Heidi Montag and then post it online to make big bucks.
Let's hope Lauren Conrad appreciates the irony.