It's the first episode where the opening titles are different from those introduced in the opening Pilot (1997). Now they also show scenes with other main actors while before the scenes only included Calista Flockhart.
Peter MacNicol joins the show as series regular from this episode (he was billed as special guest before). His joining led to the new opening credits.
It is illegal to sing "Happy Birthday" on camera, thus Ally was right to stop the singing.