The Seattle Underground is incorrectly represented as having building three stories tall and streets. There are at most only one story of a building and no streets in the Underground.
Some math problems with the dates and ages cited - the scrubbed newspaper headline near the end of the movie refers to a "144-year-old killer," which would make him born in 1829, since the movie is set in 1973, and the six sets of murders (each 21 years apart) occurred in 1868, 1889, 1910, 1931, 1952 and 1973. However, when Kolchak refers to the photo of the killer from 1926, he describes his age as "nearly 90," when it should be "nearly 100."
Several of the articles shown in the archive files of the "Seattle Daily Chronicle" clearly have the headline pasted over genuine newspaper pages by the art department; this is especially noticeable on the edition from [Tuesday] 27th March 1952 headed "WOMAN STRANGLED IN PIONEER SQUARE", which is peeling away.
Near the end of the film, the final edition of the Seattle Daily Chronicle with the "Killer Found" headline contains a misspelling. In the banner above the masthead, "committee" is spelled "committe." Interestingly, in the aborted, 20-copy-run edition with the headline "144-Year-Old Killer Found," the word "committee" is spelled correctly in the banner.
Kolchak and the researcher Mr. Beery each refer to a "photo" of Civil War surgeon Dr. Richard Malcolm multiple times, but it's clearly a drawing.
Researcher Wally Cox traces the Night Strangler all the way back to the 1870s. All of the old news paper headlines report a murder in "Pioneer Square". This name wasn't adopted until 1970 to replace the original name of the neighborhood, "Skid Row".
The "fog" in Dr. Malcolm's lair is clearly generated by a fog machine which is just out of sight at Kolchak's feet. The discharge can be seen flowing away from Kolchak quite rapidly as if pushed by a fan.