Ragueneau the baker says "Jupiter and Pluto are planets". Pluto was discovered and named in 1930, but the film is set in the 1600s.
In the beginning of the movie, in the theater when the play it is starting, a reflector with clearly electricity illuminate the actors. This was not possible in the 1600s.
The lyrics of the 1600s stage sword fight refers to formaldehyde, a compound that wasn't first reported until 1859.
Cyrano's reading glasses show a much too modern look and craftsmanship.
The Duke's letter is incriminating and he could have been held responsible for his intent to rape Roxanne but the action takes place in a very different society as today's.