The song A Diagnosis is written to be a musical theater "I Want" song where the main character sings about being currently dissatisfied and reveals what he/she really desires.
The actor who catches the envelope that Rebecca throws in 'A Diagnosis' is David Kwong- the same actor who catches the jacket Rebecca throws in the season 1 episode 1 musical number 'West Covina'
The scene where Rebecca is in the bathroom is a callback to the season's title sequence.
Rebecca's hospital room number is the same as the episode number (306).