A few years ago I arrived JFK and presented my passport to a doddery dour little immigration man. After a minute he announced that there was no visa in the passport. I almost said "Don't be so stupid", but manfully restrained myself, gently took the passport from him and proceeded to flick through the pages for the visa until I came to the back page showing a photo of - my son. We had been on a family holiday I had picked up the wrong passport on returning to work.
I thought I was in for the hefty fine plus endless paperwork, but the little man went into a back office, and returned a few minutes later, saying "It's OK, your on file", and waved me through!