In The Hobbit chapter Riddles in the dark, Bilbo asks Gollum the riddle
Thirty white horses on a red hill,
First they champ,
Then they stamp,
Then they stand still.
But people have thirty-two teeth, assuming there is a full set - and not 'thirty'!
Indeed, it would be unlikely to have thirty teeth, since the number is not divisible by four, which means that two of the teeth would (presumably, if the upper and lower jaws have the same kind of teeth) be unopposed and bite onto gum.
Interestingly, this error was missed in The Annotated Hobbit by Douglas A Anderson (2002). although I noticed, and was puzzled by, it decades ago (and plenty of other people on the internet have noticed the discrepancy).
Therefore, the riddle is - strictly, insoluble - unless we allow for congenital abnormality, dental surgery, or traumatic loss (of lesser severity than that which led to Gollum having 'only six' teeth)...
Note: I should clarify that Tolkien didn't invent this riddle - which is 'traditional'; as are most of theother riddles in this chapter.