I memorized the eye charts when I was a kid. It wasn't really on purpose - it just kind of happened. It turned out I'd needed glasses for over a year before I got them because I kept passing the eye chart test. Being a kid, I didn't quite process that they were asking me whether I could *see* the letter on the chart, not whether I knew what it was.
They got me when they transitioned over to the viewmaster-type eye tests instead of the wall chart.