I know that the person I'm stalking created a pseudonymous account on service X around time Y. Based on other information, I have a limited number of suspect accounts. The creation time leaks to me, either via a bug which would otherwise have been harmless, or because somebody writing code "can't imagine a universe where having the creation time of an item breaks your security". I use the creation time to figure out which of my candidates is actually the target.

It took me under 15 seconds to come up with that.