Author Existence Failure: Jonathan Larson, the guy who wrote the libretto and plot, died on the night of the final dress rehearsal. Which provides the perfect illustration of the show's message that you never know when your life will be over, so you should do as much as you can all the time.
Mimi "You look like you're sixteen." "I'm nineteen, but I'm old for my age!" Marquez, originally played by the 27-year-old Daphne Rubin-Vega (though, since many of the other characters are only mid-twenties, the potential exists for them as well).
The age of the cast in the film seems to add to this as well — most of the actors were in their thirties at the time, and while their ages weren't set in stone in the play or film, they certainly felt more like twenty-something "starving artists".
The 20th anniversary tour in the UK removed the line about her age entirely due to Mimi being played by the gorgeous but in no way teenager like 32 year old Kerry Ellis.
The Other Darrin: Only Mimi and Joanne are recast in place of the originals for the movie, because Daphne Rubin-Vega was pregnant at the time and obviously gained too much weight to play a junkie dancer with AIDS, and Fredi Walker felt she was too old to portray a recent law school graduate. Her one request was to keep Joanne black.