The real flaw as I see it.
SPOILERS... duh. Not gonna black out anything cause - effort.
Okay so I see a lot of people talking about thinks that aren't plot holes but either these people not understanding the time travel logic in the film or picking at moronic character choices that aren't flaws but just idiotic characters. I've seen one other good point that I thought too about how illogical it was for the bookie to risk being caught shoving a gun in the kids faces if he really believed they'd involved the police but yeah whatever.
My main flaw in the logic of this film is that Finn sees the fake picture change and then looks at the coil photo that Mr B got in advance and his decision is that because the coil didn't change then no photos could change. Problem with that being that he made the active decision to paint the coil again to save himself which surely would have made the picture not change had pictures been able to change for real... if you get my drift. Yes, it ended up being a hoax and good on him for being able to figure it out, however with the way that he had been understanding time travel for the rest of the film there was no real logical AHA! moment other than the fact the script called for it to be an AHA moment for the twist to be revealed.