As Walden is coming down the stairs, he views the mess after a drunken party and observes "This looks just like Charlie Sheens house".
Of course, Ashton Kutcher, playing 'Walden' replaced Charlie Sheen, who was 'Charlie Harper' on the show before he was fired and replaced with Kutcher.
Mikaela Hoover who appears as Morgan also appeared in another show that Charlie Sheen starred in, Anger Management.