This comment on HN by /u/jiggawatts struck me as a brilliant idea: it's an intuitive counter-example to the axiom of choice, which seem intuitively obvious, but leads to weird results like the Banach-Tarski paradox.
For those of you who are not hard-core geeks, the axiom of choice says (more or less) that if you are given a collection of non-empty sets, you can choose a member from each of those sets. That seems eminently plausible. How could it possibly not be true?
Here's how: consider the set of numbers that cannot be described using any finite collection of symbols. Such numbers must exist because there are only a countably infinite number of numbers that can be described using a finite collection of symbols, but there are an uncountably infinite number of real numbers. So not only are there numbers that cannot be described using a finite number of symbols, there are vastly more of these than numbers that can be so described.
And yet... how would you describe such a number? By definition it is not possible! And so it is not at all clear (at least not to me) what it would even mean to "choose" a number from this set.