The sort of phrasing problem that comes up when you try to present statistics in terms of small numbers:
More people are using condoms, for example, but they still play a part in only one in three sexual acts, and many of those remaining two acts could benefit from condom use, Reece says.
(See also the vaguely related 2005 Language Log entry on WTF grammar.)
Which roundaboutly reminds me that the other day I saw a news article that said something would take “fewer than three months.” Which sounds weird to me; that phrasing suggests to me that it would take either exactly one month or exactly two months, but not (say) one and a half months, because “fewer” suggests to me that it's talking about something that comes in discrete and more or less indivisible units.
The usual rule as described on a bunch of grammar web pages is that “less” is for mass nouns and “fewer” is for count nouns, which is all very well as far as it goes, but then the pages generally add that there's an exception to the rule, in that it's traditional to use “less” when talking about time. To me, that exception suggests that the usual framing of the rule is wrong; it's not so much about mass nouns vs count nouns as such, because time units (month, day, hour, second, etc) are count nouns. In my view, the rule is instead that if you're talking about discrete items as if they're indivisible, you use “fewer.”
At any rate, regardless of how the rule is presented, the outcome is the same: it's generally considered incorrect to use “fewer” with units of time. And yet, apparently people do it all the time:
Before I started looking into this, I would have said that no native speaker of English would say “fewer than three months” or “fewer than one hour.” But I would have been wrong; Googling for those phrases shows instances of each. The latter is much less common—most of the instances I saw in skimming the Google results are accidental, with punctuation in the middle—but not nonexistent. (Though the estimated number of instances for “fewer than one hour” is vastly overinflated; Google estimates 1.5 million instances, but shows only 21 if you page through the results.)
In fact, even the phrase “fewer than one second” occurs on web pages.
I wonder if these are cases of people trying to appear educated. But a bunch of the “fewer than three months” instances appear in articles in respected news venues. So it's possible that my intuition about what sounds right to people is just wrong here.
If you need to point someone to a definitive reference about the usual rule, I recommend the Chicago Manual of Style FAQ item about less and fewer, which quotes the American Heritage Dictionary's usage note. There are a bunch of other web pages out there that give essentially the same information, but Chicago and AHD are more authoritative.