Character mistake: Joan Clarke pronounces the name of the mathematician Euler to rhyme with "ruler", when in fact it is pronounced "oiler."
Character mistake: The shady gun dealer is showing off his merchandise. He picks up a handgun, and he says "it's your standard Beretta 9mm I'm sure your familiar with it..." as he's manipulating it, but he's really holding a CZ 75, a Czechoslovakian made firearm. The make and manufacture is always stamped on every gun produced. (00:40:30 - 00:41:30)
Character mistake: When he called for a dinner reservation for 12 at his home it should have been 13, as he put 13 men down. (00:32:05)
Suggested correction: Actually 14.
It's possible he simply miscounted in the heat of the moment. I'm not saying you're wrong was John doesn't seem like the type to make this sort of mistake but it could be let off as a character mistake rather than a movie mistake.
A character mistake is a valid movie mistake and this entry is listed as such.
Character mistake: During communication with ATC the co-pilot states their call sign followed by the read-back of ATC instructions. This order is incorrect. In reality a pilot or co-pilot reads back the instructions from ATC and then their call sign or tail number prior to ending their transmission.
Character mistake: Mitch states that he is Pisces then goes on to state that his birthday is 25th of March. The Pisces period goes from 19th February to 20th of March. Mitch would actually be Aries. (00:35:00 - 00:37:00)
Character mistake: When Jesperson imagines himself as a Mountie his uniform is completely wrong and is not even the right colour. As it was his ambition to join the force and he has photos of RCMP officers in his cab, it seems unlikely that he would imagine himself in a completely incorrect uniform.
Suggested correction: It's not a character mistake, for the reasons stated; it's a clear factual error from the filmmakers.
It's a character mistake since it's his imagination and, as you stated, it's not something the character would get wrong.