Frasier introduces Clint Webber to Roz as though they've never met before, but in The Zoo Story (1998) earlier this season, Roz's agent Bebe Glazer gets Roz a weekly spot on Clint Webber's show, so, presumably they should have met before.
If you look very closely, just before Frasier says "Please, everybody, nobody's perfect" while Clint is singing, you'll see presumably a stand-in at Clint's position (it is definitely not Clint's actor Billy Campbell, nor is the man even pretending to sing).