I'm sure that you know that it doesn't matter in what order you add up a (finite) collection of numbers. No matter how you rearrange them, you'll end up with the same sum. (If the order mattered, nobody could ever balance a checkbook.) This fact is called the commutative law, but weirdly enough, it's not true for certain infinite sums, called conditionally convergent series. ( is conditionally convergent if it converges, but diverges.) In fact, it's been proven that if converges conditionally, you can pick whatever number you like and rearrange the series to sum to that number. I won't give the general proof, but I will give a specific example.
Let's consider the alternating harmonic series . It converges by the alternating series test, but the sum of its absolute values is the harmonic series, which diverges. Therefore, the alternating harmonic series is conditionally convergent. Let's call its sum S. The error estimate for the alternating series test tells you that S is between 1/2 and 1 (in fact, it equals , but we don't need that here), so that it's not zero. I'm going to add S to as follows:
Summing, we get
where we have the same numbers as in the original series, except that they're rearranged so that the pattern of the signs is instead of the original pattern . But rearranging them changed the sum! (Since , S is different from .) Therefore rearranging the numbers in a conditionally convergent series can change its sum.