Yesterday I was having problems with the viewfinder on my 20D. Everything was blurry, and it was hard to see through. I examined the lenses (both front and rear elements), the mirror, and the viewfinder surfaces, and couldn't see anything wrong, so I started to assume that my eyes were just really tired. About halfway through the day I realized that the viewfinder diopter was turned all the way in one direction (I normally keep it at -1 or -2 clicks). Doh!!!! Post your dumb mistakes so that others may learn.