The faulty assumption in the more naive approach was treating operations on infinite series in the same way you would treat operations on finite sums. The order of elements being added is important, as it does change the series, and the naive approach based on putting 0 in between each numbers like 0 + 1 + 0 + 2 + ... which was incorrect. There are ways to prove it does sum up to -1/12 from what I remember though, it's just the addition of 0's that's bad.