Several kinds of fishes that are prohibited to be eaten. And several that are permitted, including contingent only on where they come from (depending on where that is and whether non-kosher fish are caught together with the kosher ones). Plus, the question of a fish that will eventually get scales, but doesn't have them now, or did have them, but lost them before now - those are all considered kosher too. Also, the story o a sliver of "hiltit," with concern or the knife, if it has absorbed non-kosher food. Plus, tekhelet - when to trust it as genuine? Plus, trusting the wives of meticulous scholars ("chaver") in the same way as those scholars are to be trusted.