Blooper: In the widescreen vesion, at 9:40 in, in the Bethesda Fountain baptism scene, right after John the Baptist sings "Everybody now!", if you look to the upper right there is a man in a pastel shirt in a green rowboat who turns his boat and heads offscreen. This is cropped out of the 4:3 version.
While the movie is supposed to take place in an empty New York City, it is debateable exactly when the people disappear from the city. There are lots of people in Central park as the principal players run down the paths to the fountain - it may only truly empty out during "God Save the People."
There's also one other instance, again right when New York is going from empty to full. As the troupe carries Jesus to the corner they finally go around and The People come streaming around - you can see a person walking on the other side of the street. However just as with the man in the boat, it's close enough to the switchover - indeed in the same scene - it's excusable.
Keep Circulating the Tapes: For decades, there was no home release of Godspell. The first VHS tape was released in the mid-90's, and was taped off Cinemax. (you can hear the "Next on Cinemax" right after the film ends!).
The Other Darrin: A few, but memorably Lamar Alford, who was replaced by Merrell Jackson for All Good Gifts, and Jackson's performance is markedly better.
Much to the surprise of folks who like seeing where actors are now, the cast of the film has had a surprising number of deaths (one third of the cast) considering such a small cast who were in their 20's at the time. By just 30 years later, in 2003:
Jeffrey Mylett (Taxi driver - Derby hat, plays the recorder) died in 1986
Merrell Jackson (Garment worker, who sings "All Good Things) died in 1991
Dave Haskell (John/Judas) died in 2000
Lynne Thigpen (Library student - Sings "Oh Bless the Lord My Soul") died in 2003