Plot hole: The grounder's leader Anya states that the missiles (flares) burned a village to the ground. If this were true, the flares couldn't have flown very far, otherwise that village would have been too far away for the grounders to know about. In a previous episode, when you saw the flares from space, it was obvious the flares traveled hundreds if not thousands of miles away.
Continuity mistake: When Clarke extracts the knife from Finn's chest, suddenly they all get thrown to the ground. Once she stands up again, in the first shot she's holding the knife with the blade pointing behind her, as it was when she extracted it, but in the following shot the blade is pointing forward instead.