The title number, "Singin' in the Rain," took two-and-a-half days to shoot and used so much fake rain that it caused a mini-drought in Culver City, California. Oh, and Gene Kelly had a 103-degree fever. (Source)

The original ending of the movie had Lina and Cosmo getting married. Yikes. (Source)

Debbie Reynolds (Kathy) was 19 and living with her parents when she made the movie, so she'd get up at 4:00 a.m. and take three buses to the studio each day. Sometimes she'd just crash at the studio, on set, so she could avoid the monster commute. (Source)

The film was a grueling shoot, and the actors literally worked themselves sick. Kelly had a fever during the title number; Reynolds had to be carried off after shooting the "Good Mornin'" number because her feet were bleeding; O'Connor was in bed for a week after that same number. Reynolds said that giving birth and shooting this film were the two hardest things she ever did. (Source)