When I see misuse of this, it makes me crazy, for some reason. The worst offenders are the ones who are trying to be meticulously correct and impose "fewer" in places where "less" is really the appropriate word.
Newspaper copy editors have come up with rigid rules of usage that lead people to abominations such as this one from the Wall Street Journal:
Fewer than three years later, Mr. Slim earned more than $500 million in profit when Verizon Communications Inc. bought the carrier...
Ugh! I can just hear the Wall Street Journal´s copy editor saying, "You should use 'fewer' with things that can be counted." The editor would be right in saying that years can be counted but would be wrong in applying the logic to this phrase. Three years is a single measure of time, a unit, not a series of three discrete units.
It should be, "Less than three years ago..."
Here´s another abomination, from a story about Olympic sprinter Usain Bolt:
It took him fewer than 10 seconds to run 100 meters but at least 10 minutes to complete his victory lap.
Again, 10 seconds is a period of time, a single period of time, not 10 individual units.
It should be, "less than 10 seconds". "Fewer" doesn´t even sound right. Again, the editor is applying the rule of countable units inappropriately.
Here is another, from the New York Times:
An $11 fare to drive a passenger fewer than four miles was all D.Y. Kim had to show for his morning's labors.
Four miles is a distance, and in the context of the story is meant to signify a single unit of measure, not four.
It should be, "less than four miles."
Here are some examples of correct usage:
Less than 1 million barrels of oil spilled from the tanker. Correct
There was more than 3 feet of snow on the ground. Correct.
There was more than 3 feet of snow on the ground. Correct.
The robbers took less than $50 from the till. Correct
After the storm, the farmer had less than 10 acres of corn available for harvest. Correct
The bridge has clearance for vehicles less than 8 feet high. Correct
I´ve visited the online AP Stylebook and reviewed its "Ask the Editor" section, where people seek advice on how to apply this rule. Most of the time, the editor applies a fierce kind of logic that results in bad advice being given. Here´s an example:
Do percentages count as numbers or amounts? In other words, is it: "Fewer than 3 percent of the country's stores..." or "Less than 3 percent of the country's stores..." – from Los Angeles on Mon, Oct 08, 2007
Editor's reply: The example seems to refer to individual stores, so fewer would be correct.
I could not disagree more. In this case, 3 percent is an amount, a quantity, a single measure, not a series of three countable units. With editors giving advice like that, it is no wonder that the abominations continue to proliferate.
As I learn every day in my struggles to speak perfect Spanish, language resists logic. For every grammar rule that exists, there are other rules that countermand it. We need fewer editors sticking to the logic of a single grammar rule, and more understanding the context of the rule.