There was an article a few years ago that found that about 1/3 of passengers carring cell phones inadvertedly left them on through the entire flight. This could be because the pushed the worng butto to turn them off, failed to make sure they were off, etc., etc. I carry at least two cell phones and I can assure you at times I have accidentaly not turned them both off.
Short of requiring everyone to turn in their cell phones at the Xray machines and giving them back to them on arrival at their destination (obviously impossible logistically), there will always be cell phones and other electronic devices in the 'on' position on an airplane during flight.