The distinction between less and fewer is one of the most popular rules in the peevers' arsenal. It's a staple of lists of grammar rules that everyone supposedly gets wrong, and sticklers have pressured stores into changing their signs from "10 items or less" to "10 items or fewer." Students have it drilled into their heads that fewer is for things you can count while less is for things you can't. But there's a problem: the rule as it's commonly taught is wrong, and it's dulling our sense of what's actually right.
Take this recent post by Lucy Ferriss on the Lingua Franca blog. She came across a sign that warned that flushing certain items would "mean one less toilet for you to use until it is fixed." Ferriss mused about whether this was right or wrong, noting that although toilets are countable, "to write one fewer toilet feels unnatural, since fewer seems to want to modify a plural noun." Indeed, as Bryan Garner writes in his Modern American Usage, "less applies to singular nouns or units of measure," while "fewer applies to plural nouns" (emphasis mine). Garner calls one fewer "an awkward and unidiomatic phrase" and concludes that it "is a kind of hypercorrection induced by underanalysis of the less-vs.-fewer question."
But even Garner underanalyzes the question, presuming the use of less with plural nouns to be a modern problem. But people have been using less with plural nouns for as long as English has existed. Alfred the Great, the king of the Anglo-Saxons and a prolific writer and translator, wrote around 888 AD, "Swa mid læs worda swa mid ma" (with less words or with more). He didn't follow the rule because it hadn't been invented yet, and it wouldn't be for almost another thousand years.
It wasn't until 1770 that Robert Baker suggested that maybe people should use fewer instead of less with count nouns, and the rule has expanded and become more rigid since then. Though many grammarians since Baker have promoted the use of fewer with count nouns, most have acknowledged that there are many exceptions to the rule, though even these are starting to disappear. Less is used not only when following one but when describing physical quantities or units of measure, as in less than 50 miles or less than 2,000 pounds. Constructions like 25 words or less and less than 100 people used to be standard too, but now 25 words or fewer and fewer than 100 people are gaining ground, despite sounding a little stilted. Fewer is even popping up in constructions like fewer than 5 percent and fewer than half of all Americans, which sound not just stilted but downright wrong.
A rule that has so many exceptions and that leads so many people to create such awkward constructions must have missed something in its formulation. So let's look at the evidence and see if we can discern what the real rule is—the rule that native English speakers have been following for more than a millennium, despite what they may have been taught, and the rule that they followed before they all became self-conscious and started using overcorrecting to fewer.
As we've already seen, fewer has to be used with not just a count noun but a plural count noun. But fewer tends to be avoided when the noun is implied, as in 10 items or less. One traditional exception also says that less is used with units of measure, but actual usage shows that fewer is usual choice when the unit immediately follows fewer, as in fewer miles or fewer pounds. But as noted above, constructions like less than a hundred people used to be considered standard. That is, it's not just units of measure that use less—it's nouns that are separated by than. So rather than say that there's an exception for units of measure, as in less than 50 miles, and that there's an exception to the exception for fewer miles, and that there's also an exception for regular count nouns when they follow than, the rule becomes a lot simpler if we say that fewer needs to be followed immediately by a plural count noun. Otherwise, use less.
But this still doesn't explain why people sometimes use less followed by plural count nouns, from Alfred the Great's less words to today. It's easy to say that even the best of us make mistakes, but remember that this rule didn't even exist for the vast majority of English's existence. The simplest answer is that less and fewer are in variation and always have been. Just as more can be used with both count and noncount nouns, so can less. In other words, less is the default, but we also have the option of using fewer when it comes immediately before a plural count noun.
Since Baker first codified the preference for fewer with count nouns, it has come to be associated with a more formal style. Though some grammarians have made Baker's rule more rigid over time, you're still free to choose. Be aware, of course, that using less immediately followed by a count noun may raise some people's eyebrows, but also remember not to take things too far in the other direction and use fewer in places where it's unidiomatic. Despite what the sticklers may say, there's nothing wrong with less than a hundred people or 10 items or less. This is one less thing for you to worry about.