Did anybody fall for this in 1967? In that year I was eight years old, and I already hated this kind of crap. I would have been yelling at the screen. I yelled at the screen last night.
Perhaps this was the first heist film that used laser beams in a vault as an obstacle to thieves, but why did they do it so badly? The laser beams, which criss-cross the vault like a spider web, are done in an ostensibly clever way: translucent tubing filled with light. But when the thieves climb over the beams with a fancy telescoping ladder rig we can clearly see the laser beams sagging! Not just a little, either. Worse, we can see a connection point where two pieces of tubing were joined. It's a friggin' close-up! This kind of sloppy craftsmanship really takes you out of the film.
It gets worse. The safe is rigged with a delicate noise detector. The sound of a cigarette lighter is enough to set it off. The solution? Lift the entire safe with pneumatic lifts, stick on little wheels, soundproof the wheels with shaving cream (I kid you not), and push the safe ten feet away from the sound detectors. Then start drilling those titanium doors. Then blow it with nitro glycerin. Then silently push the safe back up the ramp and into the vault (more shaving cream), disconnect the pneumatic lines, cart away your seventy-five pounds of equipment, and close the vault door. All without making as much sound as a Zippo.
This film was co-produced by Spanish, German, and Italian film companies. Is it possible that in an audience of, say, 100 Spaniards or Germans or Italians, no one made a huge PUK-SSHHH! sound when those air hoses were disconnected? Maybe not. Maybe they were better at suspending disbelief than an eight-year-old American.
Compare this to the new gold standard for technical competence in that era of film-making, "2001: A Space Odyssey." Although history has unspooled very differently than as predicted in "2001" (no cities on the moon, no manned exploration of other planets), those cinematic predictions were very carefully executed. The craftsmanship was exquisite. If the Americans and British who made "2001" had been as clumsy as the hacks who made "Grand Slam," there never would have been any discussion about the religious or spiritual meaning of that bizarre last act. Those questions were discussed very seriously in the late 60's (and still are) because "2001" was a believable world where our powers of disbelief remained suspended for 2 1/2 hours.
Now, of course "Grand Slam" is just a heist movie. It's doesn't have any deep pretensions. Does that excuse its technical shoddiness? Of course not. Even a frothy story needs to keep us within the walls of the story, so that we can be lied to convincingly. When fundamental facts are ignored, the movie is over. The Confederate army can't wear blue. You can't drive to Australia. And laser beams can't sag.
Maybe this is why the United States was the technological powerhouse of the world in the 1960's. We cared about getting it right. And we still do. Even bad American movies are produced with a technical brilliance that outstrips the stupidity of the above-the-line talent. And web sites like MovieMistakes.com help keep our standards from flagging.
Maybe "Grand Slam" deserves credit for inspiring better films, such as "The Italian Job" two years later. Perhaps you can insist that "Ocean's Twelve" owes its dancing laser beams to "Grand Slam." But at least in "The Italian Job" when things blow up they go BOOM!, and in "Ocean's Twelve" the laser beams don't sag.