Continuity mistake: There's a scene where Audrey Hepburn's husband Sam is leaving their apartment. Standing on the pavement outside he hails a cab, which pulls up through a (quite sizeable) puddle, splashing him with water. Opposite him on the other side of the road are parked two cars and the villains' Volkswagen van. There's a quick cut-away exchange of "pleasantries" between Sam and the cabby before it drives off. The next shot has the villain (Richard Crenna) emerging from the van, crossing the street, and making for the door to their apartment, only all of a sudden the puddle has disappeared, as have the two cars which were parked behind the Volkswagen van.
Plot hole: The neighbor girl, using a pre-arranged code, rings Audrey Hepburn's apartment twice when Richard Crenna leaves the phone booth across the street. Hepburn thinks she now knows that Crenna is mixed up with Alan Arkin's strange mix of strange friends who have come to get back the long missing heroin-filled doll. However, anybody could have been coming out of the phone booth. It strains logic that Suzy would make that deduction. (01:25:00 - 01:31:00)
Factual error: When Susy reads the phone number in Braille, what is written on the paper is gibberish. Not only is it not a number, it says "shchsting."