What Could Have Been: Numerous endings were considered. In an interview, Michael Caine said that one was balancing the cliffhanging bus by burning off all the fuel only for the Mafia to turn up and reclaim the gold. Others included the gold being safely transported to Geneva or getting all the way to England, only to be returned to the Mafia as part of a compromise.
A few original lines were later taken out of the movie when the director realized that not many non-Brits would understand them, one regarding "someone from the smokes sticking his bugle in" on Charlie's operation. In American english, the line means that someone was worried that someone else from London would stick his nose into their business.
Coventry does 'a Toronto' - the sewer chase scenes are all filmed in Coventry, which at the time was having its pipes replaced, meaning easy access for filming.