Photographers often read significant things in their images after the fact. It's only when they look at the image that they spot something they didn't see it at the time.
Many say it is and it proves that photography isn't on the same level as other visual arts; a painter couldn't paint something "by accident". But I'm not so sure.
When I took this photograph in Clerkenwell, I knew that I wanted the shallowest depth of field I could manage, and I knew I would focus on the tatoo. I saw afterwards that the gradation of focus from the tatoo, through the girl's face to the defocused background was interesting.
Of course I knew what I was doing!