The storyline surrounding the song "Defying Gravity" was based on Chris Colfer's real-life high school experiences. He repeatedly asked to sing this song in the school talent show, but was always rebuffed because the song was written for a female character. After venting about this experience to Glee's creator/writer Ryan Murphy, it was written into the show.
Lea Michele apparently was prone to flipping the chair backwards and hitting her head. Amber Riley at one point caught the edge of ramp and fell off completely.
Zach Woodlee has stated that the Proud Mary number was the scariest number to produce as of release.
Some of the actors had trouble learning to distribute their weight properly, which resulted in some of the chairs being flipped backwards.
The ramps used for the Proud Mary number were originally built too steeply and had to be rebuilt.