I am guilty. Yes, I, the Word Doctor, the professed Grammar Guru, have been guilty of using the incorrect word in a context. **gasp** Shocking, I know. But I never claimed to be perfect!
I caught myself speaking with someone about having less tasks to do than usual. Something felt odd, but it wasn't until later that I realized what. The use of the word "less." I should have said "fewer"!
You see, our language is wonky (as if you didn't know). More is more, no matter what the context. More time, more money, more to do, more pencils, more cats...it all works.
But in the "less than" category, there's a difference. Less refers to a smaller amount of one thing, or one insubstantial. You can have less time, less money, less to do, less hair, etc.
Fewer refers to a smaller quantity of several items. Fewer people, fewer pencils, fewer cats, fewer hairs on the head, fewer marbles.
Get the idea?
As with so much in modern English grammar, this is a relatively minor transgression, but can be an important consideration when writing something meant for general publication - a book or an article, perhaps. In these more professional settings, understanding the difference will lead to fewer refusals and less stress!