Fowler's dictum holds for many other prescriptive rules of grammar. In many cases, ignorance is bliss. Your writing will probably be better, because you're not twisting yourself into knots trying to avoid some illusory error, and you won't waste your time thinking about other people's grammar when you should be attending to their meaning.
I'm particularly prone to the second case. I have a nasty habit of noticing not just when people violate a rule but (for certain rules anyway) when they observe the rule. If you think about it, that's pretty perverse. When someone observes subject-verb agreement, we don't normally note the fact, unless perhaps we're teaching a second-language student with limited proficiency. But with more recondite rules, I find myself noting when a person observes them, for example when someone avoids a split infinitive.
As a case in point, consider this morning's XKCD comic:
The title, which displays when you hover your cursor over the image, reads "Programming the sexbots to enjoy sex seemed a sensible move at the time, but we didn't realize the consequences of their developing fetishes."
What threw me about this sentence, grammatically speaking, is the phrase "the consequences of their developing fetishes." Based on the comic itself, I think the most logical way to understand Munroe is that we should interpret him as saying that we didn't realize the consequences that would follow after they developed fetishes. In other words "developing" is a gerund-participle and "fetishes" is its direct object.
If that's the case, Munroe is following the prescriptive rule which insists that the "subject" of the gerund be in the genitive (i.e., possessive) case. The ordinary construction under this reading, the one uncontaminated by prescriptivism, would most likely be "the consequences of them developing fetishes."
Whether you think of the version with "them" as an error or not, it has the virtue of avoiding an ambiguity, because the other way to understand "their developing fetishes" is with "fetishes" as the head noun and "developing" functioning as an adjective. That would make the phrase mean something slightly different, which I would paraphrase as "the consequences of their fetishes, which are developing."
I can rationalize this interpretation too, although I think it's substantially less likely than the first.
If, as I suspect, Munroe had the first meaning in mind, then he was practicing prescriptive avoidance and created an unintentional ambiguity. But even if he had the second meaning in mind, it's still a sentence that messes up those of us who have had enough exposure to prescriptive rules to notice such things.