It often amazes me just how lucky you have to be to find some bugs. I've found bugs when you use data that may only affect 1 in 1000 items in your database. I've found bugs that only manifest on certain days, if I'd been testing a few days earlier, or a few days later I never would have seen it.
Once you see these things for the first time, you can add to your list of oracles and that should help you find similar bugs elsewhere. Which goes to prove the old addage, the more you practice, the luckier you get.