Misplaced face-down card
This actually happened to me, as a player, at a PTQ at Competitive during Morph's first go round. I cast a Morph, passed the turn, picked up my hand, and saw the morph in my hand (it was the only one.) I called a judge immediately, while my opponent looked on puzzled. When the judge arrived, I explained to the judge what had happened, and even flipped up the “Morph” revealing that it was a Fever Charm. What happened next astonished me.
I got a warning, and was allowed to switch the two. I imagine this was because I called the judge myself, revealed the error (expecting a GL,) and could not benefit, and since there was only one card with Morph in hand, it was obvious what I was planning to do.
At FNM, I would probably do the same thing, only if there was only one card with morph in hand. Otherwise, I'm not certain how I'd handle it, but reveal/substitute seems most reasonable.