I have seen here and there people try to use the wrong side of a hammer, usually either small children or people with no experience at all using hammers. But the second you show them the right way it sticks: the sides are visually different and, more important, the right side works better.

What I've never seen in my life is a random group of 9 people (per the study) where all of them consistently use the wrong side of the hammer repeatedly across four sessions.