Here's an example. Patent lawyers absolutely love the word "plurality", a very uncommon word in normal English. They mean "more than one", but why don't they say that? It's not only clearer but it's also less ambiguous (check the many definitions of "plurality").
It's because they are copying a rule without thinking about it. Other examples of that:
* "an historical occasion" - this is wrong, it's "a historical occasion", but people hear it and think it's some special rule
* Not using "I" in research papers, even with a single author! There's no rule that requires this but people think there is. I read a paper once that used "I". It was great.