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When switching from a high-end compact camera, on which I usually composed shots using the screen, to an SLR, on which it's mandatory to compose shots through the viewfinder, I often found myself making one particular kind of error.

Looking at a screen is looking at a picture. Looking through a viewfinder is like looking through a telescope or binoculars: it's closer to looking at the real world. On a picture, it's easy to spot if the picture is crooked: the horizon doesn't line up with the top and bottom of the screen. If you hold a printed photo wonky, your sense of balance tells you the horizon doesn't agree with which way is up, and you turn it so it's right. It's easy to take non-wonky photos using the screen.

But if you are looking through a viewfinder, the horizon can't be wonky. The horizon's always in the same place: only the camera can be wonky. When I started using an SLR, I kept taking photos with wonky horizons for this reason. It took me some time to train myself to check whether the camera was straight, not the horizon.