“Song Beneath the Song” — more casually known as Grey’s Anatomy ’s musical episode — aired on March 31, 2011, and to this day it allegedly remains one of Shonda Rhimes’ favorite episodes of the medical drama. That’s likely why it still bothers her that the higher-ups were iffy on the concept at the time, as Rhimes told EW:

Grey's Anatomy, now the longest running primetime medical drama in American television history, was well into its seventh season — and therefore had nearly 150 episodes under its belt — when it decided to do a musical episode.

Grey's Anatomy creator Shonda Rhimes speaks to whether she would return to the medical series to write the eventual series finale (if that day comes).