George Clooney returned after having left the show years ago for one episode, the title "old times" refers to that. His return wasn't advertised by the network, that was a special condition Clooney required for his return.
Ernest Borgnine's character is named Paul Manning, a tribute to the ER writer and producer who passed away in 2005.
George Clooney made cameo appearances in two episodes after he finished his regular run playing Dr. Doug Ross. Clooney made the same deal for both episodes with the show and NBC: he would return at scale pay if his appearance was not used in any commercials or other promotions for the episodes.
In one scene, the iconic 'Doug and Carol theme' from the show's first seasons is heard.
Carol Hathaway expresses a dislike of Thursdays to Doug Ross. An inside-joke to NBC airing the show on Thursdays.