The big taboo was called a "jump cut."
Most of you know the term, but for those who do not, let me explain. A jump cut is something that cannot physically happen. For instance, if you go from from a shot of Senator Jones at his desk and cut directly to him shaking hands on the campaign trail, that is a jump cut. In effect, he has "jumped" from the desk to the campaign.
Jump cuts don't have to be that drastic to qualify. You might go from a shot of the Senator on the phone and cut directly to one of him with his hands folded. His hand has jumped from the phone to his other hand.
Hence the creation of "cutaways." The CBS guy taught me the importance of having shots to "cut away" from your subject; something to put between shots to avoid a jump cut.
The only other way to get around this was a dissolve, which implies a time change, and back then it took an act of congress and a friendly director to get a dissolve made. It was too much time and trouble, so we simply shot enough cutaways to deal with jump cuts.
Why the rule against jump cuts? Well, they're jarring to the viewer. And since we want to make our product flow seamlessly, we avoid anything that's jarring.
So it puzzle me that I'm seeing so many jump cuts on resume tapes these days. I have to assume that half of you don't know about them, and the other half don't take the time to throw in a dissolve. Which, in these days of non-linear editing, takes about ten seconds.
If you've ever seen a great sequence in which all the shots match up and it looks like it was shot with two cameras, that's the result of a photog painstakingly trying to avoid a jump cut. He might tell the Senator on the phone to freeze while he changes the angle and gets him hanging up the phone. That's a small sequence, which is another way to avoid a jump cut.
Some NDs don't care about this, but many do. You can impress an old school ND who values production values by keeping the jump cuts out of your stories. It's so easy to avoid, there's really no excuse.