I was working on the problem Xenia and Tree here: http://codeforces.com/problemset/problem/342/E
But the problem statement doesn't actually matter.
My first solution gets WA: http://codeforces.com/contest/342/submission/33140735
and my second gets AC: http://codeforces.com/contest/342/submission/33140905
The only difference between the two submissions that mattered is that I increased MAX_N from 100000 to 100005. This should not have changed anything, because I'm pretty sure my array sizes were all large enough. So why did this happen?