Beautiful Mistakes: Serendipity

Sometimes you focus your lens on one thing but end up with an image of something entirely unexpected. It may be due to an incorrect setting (e.g. mis-focused) or something happening within the frame that you couldn't anticipate (e.g. a new element entered the frame). For this challenge, describe the shot you had intended and explain how the resulting image differs. This is not about how the subject suddenly moved or changed (e.g. the subject made a funny face at you), but how something totally fortuitous occurred. Important, you MUST provide an explanation or be disqualified (see additions rules).