I wear trifocals. My Canon has a small viewfinder. Focusing is an adventure of guesswork. I shoot and hope for the best.
Consequently, in my macro work, I often do not see what a frame really contains until I download the image and see it in Bridge’s preview. Many of the icky little bugs that appear in much of my work are accidentals. I never saw them.
That’s the case with this spider. It is unbelievably tiny. I was focusing on the foliage — whatever the hell it is. Juniper? Eastern red cedar?
Photography is filled with accidents and luck. But I’d argue that the difference between a consistently good shooter is timing, patience, and especially anticipation. That’s what being a sports photographer taught me. Now I go looking for — anticipating — what I cannot see through these damn trifocals.
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5280 Lens Mafia 
This is an awfully nice accident. I know what you mean, too – I usually have my reading glasses on so I can see the meters and screens on my camera, but then I can’t see in the viewfinder with them, and as a result I’m not always sure what the hell has happened until I get them downloaded.