Symmetrain is more than a clever pun
It also combines 'spot the difference' puzzles with a kind of endless runner experience. I know that doesn't sound too pleasant on its surface, but have a look at the trailer, and like me, I think you'll be...er, plesantly surprised.
As you can see, it's got a really nice almost storybook art style, and looks like a quite relaxing experience... as you're straining your cognitive functions to spot everything that isn't in perfect alignment. The game is availble now on the App Store and will set you back $0.99.