Inexplicable errors in films are nothing new - they've been around for as long as the movie business has. Case in point - "North by Northwest", released in 1959. Who could forget that climactic shooting scene at Mount Rushmore, where Kendall shoots Thornhill with a gun loaded with blanks? The plan wasn't as top secret as they both thought it was - there's a child in the restaurant who knows all about it.
Watch the scene again, and as the blank-bearing gun is pulled, there's a boy in a blue sweater in the background who's covering his ears before the shot is even fired. He knew it was coming! If their scheme was so lax that even children knew about it, it's no wonder they got rumbled. It's a wonder that they ever made it through to the end to take that suggestive trip into a train tunnel.