When the waiter guy gives the tray to the bad guy, the bad guy is in a suit with a regular tie. Then, when he goes in to Leroid's room, he has a bow tie instead of a straight tie.
I agree with all the slip ups pointed out by the users, but the biggest slip up of all has surprisingly been missed by all. That slip up is the atrocious and very painful pseudo Afrikaans accents employed by Joss Ackland and Derrick O' Connor (among others) in this film. No Afrikaner speaks English like that, surely? PS: My mother is an Afrikaner, by the way.