Other mistake: In his search of Giselle, when Edward knocks on doors 209 (hissing cat), 405 (barking dog), and 714 (purple guy), the funny thing is that in all three close-ups the three door jambs/trim have the same paint glob markings. The actual doors were changed, but the jamb/trim remained the same for all three shots. (Only visible on widescreen DVD).
Other mistake: In the song, Happy Working Song, that Giselle sings while cleaning the apartment, she references cleaning the hair clog from the "shower" drain. But in the bathroom scene shortly after, she doesn't know what a "shower" is, so she should not have used that exact term earlier.