Errors in geography
The view of the Seattle skyline from Frasier's high-rise condo is an impossible view. The angle of the view, with the Space Needle in foreground and the skyscrapers in the background, is from north of downtown Seattle looking south. There are no high-rise condos north of the Space Needle, just one very massive hill, Queen Anne Hill. In order to get the view that he has, Frasier would need to have a ground-level house about halfway up the side of Queen Anne Hill.
In the 2000 season premiere, Donny, a Seattle-based lawyer, launches a breach of contract lawsuit against Daphne Moon for backing out of their wedding. It is noted that the suit sought "$100,000 in punitive damages". Washington State does not allow for punitive damages in its courts. (Only cases heard in Federal Court are eligible for punitive damages in the State of Washington.)
Incorrectly regarded as goofs
As always with any long-running comedy series, there are some inconsistencies in characters' back-histories between episodes, but we generally excuse them. Discrepancies between this series and Cheers (1982) (from which it is a spin-off) are completely exempt from nitpicking (and some have been explained away, anyway).