When Elaine first arrives at the City Agencies office, watch the display board which lists the names of all of the departments within the building. While the departments should be listed in numerical order, the office for the Public Transportation department (#615) is inadvertently listed below numbers 669 and 689, rather than before them. Note that the "Transportation" in "Public Transportation" is misspelled. The "T" and the "A" from the word's third syllable are missing, and the word written on the sign actually reads "TRANSPORTION". Also note that while AGING is spelled "Ageing" in Australia and places overseas, the Americanized version of the word is correctly spelled "Aging". While few places in the USA do spell it "Ageing", It might have been assumed that spelling it correctly would confuse certain people as to it's correct pronunciation, and thus it was spelled phonetically...
The Wessex Hotel throughout the movie is revealed to have sixteen floors, with a decorative freeze covering the sealed off 13th floor. However in the initial shot of the outside of the hotel at the first of the movie, only two floors can be seen above the freeze covering the 13th floor, when there should have been three, the 14th, 15th, and 16th.
In the opening elevator scene, it is explicitly said that the hotel has no 13th floor. From there, the film's entire premise centers around the 13th floor being allegedly nonexistent. However, late in the movie, the stairways are revealed to have open access to all floors, which means it would've been impossible to conceal a stairway entrance to the 13th floor. Even if the 13th were blocked off somehow, anyone could walk up from the 12th, or down from the 14th, and see that the 13th did indeed exist.