The number "Heavenly Music" was originally a trio with Gene Kelly, Phil Silvers and Judy Garland. Garland didn't show up to work the day of shooting and they filmed the scene without her.
One one particular day of fiming, Judy Garland was said to be "not in a fit state to work" so Gene Kelly feigned a fall so that she would be able to take the day off.
Judy Garland was supposed to make one more MGM musical, Royal Wedding (1951) with Fred Astaire, but was fired from that production, making this her MGM swansong.
The number "Get Happy" was filmed several months after the rest of the film. During that time, Judy Garland lost considerable weight, which led to a longtime legend that the number was shot for a different film. In fact, Garland had worn the exact same costume in a deleted musical number from Easter Parade (1948), which also contributed to the false legend.
This would be the last time that Gene Kelly and Judy Garland were paired together onscreen.
Judy Garland is said to have been at the height of her drug addiction throughout filming, resulting in her weight changes, mood-swings, and unexplained illnesses. It was due to this behavior that MGM fired her after filming completed.
The final of three movie musicals in which Judy Garland and Gene Kelly starred in together, though they did appear separately in other movies and features.