The exceptional fortuitousness of finding useful software for an obscure purpose.
We researchers build a lot of things that no one will ever use. Why do we do this? It's not because we like to suffer, but rather because often we need solutions for very particular problems. So imagine my joy when I discovered an excellent piece of software that was written for a different purpose but it works shockingly well for my purpose. At a meeting at the Weizmann with a bunch of folks from Konstanz I was chatting with Ian Etheredge and he pointed me to it.
Take home messages: Talk to people. Someone may have built exactly what you need!