When Wallander visits the comatose Hoglund in the hospital, a nurse stops a cassette recording of a child singing "Incy Wincy Spider." Although the cassette is stopped in the middle of the word "spider," when Wallander presses Play again, the song starts from the beginning.
There are repeating mistakes when Polish language appears onscreen. Most notably, when Wallander and Lindeman are checking CCTV records. Ferry captain helps policemen to find proper file, saying supposedly Polish name "Czekanie sali" (the same tag for the video is seen on the computer monitor). Although both words are Polish, together they're gibberish. It seems like literally translated English term "waiting room" into Polish: "waiting" is "czekanie", and "room" is "sala".
Names of the both young Polish girls (the one killed on the ferry and her prostitute friend), aren't Polish at all. "Snezna Botez" is completely nonsense (despite there is a Polish word "sniezna", but it's not human first name, it means "snowy"). Only surname of the other one - "Nowak" - is common in the Slavic countries like Poland, Czech Republic, Slovakia and others.
After Jan Petrus struck Wallander's colleague with the sledgehammer it was clear that either the hammer was a lightweight prop or he had improbably strong wrists by how he held the sledgehammer extended at full length before him.
There are a lot of typos in Polish names on the passengers list, when vendor in the duty free shop shows it to the policemen on the terminal.