ER got a visit from Kumar himself, Kal Penn, in a 2001 episode when he guest-starred as Narajan, while it was Harold, aka John Cho, who stopped by Grey's in 2006 to guest as Marshall Stone. And no White Castle was consumed during either appearance.
Before he began breaking tween hearts everywhere as High School Musical's Troy Bolton, Zac Efron appeared in a 2003 episode of ER as Bobby Neville, a shooting victim who didn't make it out of County General alive. Meanwhile, a year into her Sonny with a Chance run on Disney Channel, Demi Lovato arrived in a 2010 Grey's episode to show fans that she could handle material darker than anything the Mouse House would ever let air. She played Hayley May, a young diasnosed paranoid schizophrenic who had tried to claw her eyes out.
Before Mariska Hargitay became the iconic Det. Olivia Benson on NBC's long-running Law & Order: SVU, she appeared in 13 episodes of ER in 1997-98 as Cynthia Hooper, a County General desk clerk who developed a relationship with Anthony Edwards' Mark Greene. Meanwhile, before she became the mother of the Big Three on This Is Us, Mandy Moore appeared in a handful of 2010 Grey's episodes as patient Mary Portman, who survived the intense hospital shooting only to not survive her colostomy bag reversal surgery six months later.
Before she was giving us life as one of the Real Housewives of Beverly Hills, Kyle Richards appeared in 21 episodes of ER from 1998-2006 as Nurse Dori. Meanwhile, as Real World: San Diego participant Jamie Chung made her transition to acting, she appeared on both shows, visiting ER first in 2007 and Grey's next in 2010.