"The secret impresses no one. The trick you use it for is everything"
- Alfred Borden in The Prestige
The spinning totem at the end of Inception is the most obvious example, but take another look at the above shot which appears in a flurry of moments before the end of Memento. We've just received the truth of Leonard's situation in one large outpouring from Teddy. But just as we think we've got everything squared away this shot throws one last wrench in the gears. There is simply no reconciling it.
It must take place after the events of the movie because that spot on his chest has been blank the whole time. But what is Leonard's wife doing there if Teddy was telling the truth about Sammy Jenkis? And if this is a flashback and he simply had the tattoo removed (an explanation I consider stretching it) then why does he have the familiar JOHN G RAPED AND KILLED MY WIFE tattoo if she is still alive?
I've always took the shot to be an expression of Leonard's memory getting jumbled midway through the process of reshuffling itself into an order more to his liking. Given that explanation not everything has to fit. But once you've pulled on the thread of certainty everything starts to unravel. How reliable was Teddy's explanation, really? He undoubtedly told a lot of truth and he had the polaroid to back him up, but 100% true? Doesn't seem likely, does it?
Take the audience right up to the edge of understanding but stop short. Give them something that lingers in the imagination. That's the way to do it.