When Mary looks through the phone book in the post office, the name reads M J A Horowitz. When she writes the first letter, she writes to M J Horowitz. However, when she mails the first letter, the name on the envelope she posts is "Max Horowitz".
During the first letter Mary writes to Max, she is left-handed. When she's writing all the other letters, she is right-handed.
When Mary sees all of her letters on Max's wall, there are letters which are exact duplicates of others.
When Max is seen in court the judge is seen to be wearing a old powdered wig, back dropped by an American flag. This wig however identifies a magistrate operating in a court in a country that is part of the British Commonwealth (such as Australia, Canada, Britain, etc) and is not worn by American judges.
The New York Times is depicted in the film as a tabloid akin to the New York Post, lacking its signature nameplate and typesetting.
Max's typewriter has incorrect letter positions. Not a QWERTY key layout.
When Max is in the hospital, the sign over his bed says "Nil By Mouth," a medical instruction which, while routine in Commonwealth nations, is unknown in the US.
While Max is typing on the typewriter, many times the carriage moves in opposite directions. But while typing the carriage should move only from right to left.
When Mary flips open the phonebook at the Finkelstein entries, the same 7 lines are repeated. When she points at the entry for Horowitz (having not flipped the pages at all), 6 of the same 7 addresses are used (all but the one for M J A Horowitz); only the names have been changed. The one that is changed is from Herbert to Hubert and inexplicably leaves an extra blank space between Hubert and Street.
Max is diagnosed with Asperger's Syndrome in the late 1970s, but the diagnosis was not brought into the Diagnostic and Statistical Manual until 1981 (though since Lorna Wing invented the term Asperger's Syndrome as early as 1976, this is not an impossibility.)
After Max is admitted to the mental hospital, one of the medications he is prescribed is Zoloft. However, Zoloft was not introduced to the American market until 1991.
The eagle head logo shown on the U.S. mail box came into use in 1993. Before that (i.e. during the time in which this movie is set), the full eagle logo was used.
When Mary is imagining what life is like in America, she imagines a can of "Yanky Cola"; however, the can has a retained ring pull, which wasn't first used until 1977.
Mary's supermarket is shown using plastic bags in 1976. Supermarkets in Australia still used paper bags then, and plastic grocery bags did not come in until the 1980s.
Part of the movie is set in Australia and part of the movie is set in New York, however, the cars always drive on the left side of the road, even the scene in New York.
When the story about the character (Max's upstairs neighbor's friend) who buys a Ferrari is told, the car is shown as having right-hand drive. It's extremely unlikely someone in the US would buy a right-hand drive Ferrari, although of course that would be common in Australia where the movie was made.
In the U.S., a can with the label "tinned spaghetti" is shown. While this wording would be used in Australia, it is rather unlikely in the U.S., where "canned spaghetti" would be the preferred term.
When Mary has the printing run of her study of Asperger's Syndrome destroyed because of Max's disapproval, the local paper headlines "Local Academic Pulps Novel and Career." Her book, however, was a non-fiction, academic study rather than a novel, which is by definition fictional.