The Hollywood Code and some plot problems...spoilers
In general I think the old Hollywood Production Code was a good thing for society...for you youngsters, it was forbidden to show someone profiting from a crime.
However, I also dislike it when such constraints lead to sudden changes near the end of an old movie which often don't make much sense and were obviously contrived simply to fulfill the requirements of the code.
Case-in-point, the serialized money. I find it hard to believe, with all the thought Paul Mason put into covering all the bases regarding the heist, and actually having handled the money while still in the vault, that it would only be after they had successfully stolen the money and were about to divide it and make their escape, that he noticed these were all new, serialized bills, and therefor would be very difficult to spend or launder.
It didn't ruin the movie, but it seemed completely tacked-on to the end to make the movie conform. Has anyone read the book? I'll wager they get away with it there!
Another minor plot hole was when Paul and Louis had removed the money, gone through all the trouble to get it into the office, and then were again on the ledge looking almost directly down onto the Mediterranean. If this truly was a well-planned crime, why not skip the elaborate way they got the money out, and simply place the money in a water-tight bag with flotation sewn into the lining and throw the bag down to a waiting boat?
The special bag could have been smuggled in under their jacket as part of a "fat suit".
Granted, this takes 95% of the fun out of the end of the movie, but it should have at least been addressed by having Paul dismiss the idea when made by another character by saying something like: It is too far to throw the bag, it might become caught in the rocks half-way down, or the police patrol the bay.... or something. I would have let it go but they showed the nearly vertical drop several times and had Louis actually mention it too. If your going to build into the story a nearly vertical drop for danger and suspense, you should consider that it might also provide the criminals with an advantage.
And finally, if you were Eli Wallach's character of Poncho, why not simply make the Professor (Robinson) concoct a pill for himself, or perhaps a dog or a cat, and test it to make sure he knows what he is doing. I would want to see it work at least once before I tried it myself.
And, this wouldn't ruin the plot because he could still get cold feet, just like he did.