Aside from the problems already mentioned, I dislike the coin/powerup system:
1) Coins are only given out if you improve on a previous score, and then the reward is the same regardless of the size of the improvement. Got an awesome, unbeatable score first time round on a level? Tough, no more coins for you. The worse your score, and the more slowly you improve on it, the more coins you can "milk" out of a level. Great way to turn a puzzle game into a grindfest.
2) Powerups are permanently gone once used. Given that the supply of coins/powerups is limited, and that they're necessary for some of the awards, you'd hope that at least they'd still be active when revisiting the level you used them on.
Defocus or cross your eyes (as if you're trying to view a 3D stereogram) so that the two copies of the picture merge into one in the middle. Some people find this hard to do - sitting further back may help. Now the differences will show up as blurry regions, where one eye is disagreeing with the other. Think this is cheating? You're still finding the differences using only your eyes.
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 "Moonlight Differences"){kind=icon}