The People vs. the Fonz - S11-E11
Factual error: When the Cunninghams are talking about Fonz's trial, Mr. Cunningham can be seen wearing a digital watch. Digital watches didn't exist in the 50's.
Uniform Day - S6-E17
Factual error: It is Uniform Day where they all have to wear their uniforms. The badges they all wear are those of patrolmen. The problem is that they are detectives and the NYPD has detective badges, which are distinctly different looking.
Horror in the Heights - S1-E11
Factual error: Outside the Indian restaurant, the street curb is painted red. In Chicago, throughout the 1970s, curbs were painted white or yellow to indicate special parking zones; never red.