When I was in high school (30 years ago), I took a biology class, and the instructor showed us an Ishihara color test for color-blindness. (This is the "hidden numbers" test.)
What I thought I saw once or twice was two different numbers* that had been embedded in the same slide. Were there slides in the test that had two or more numbers, or was I hallucinating?
[EDIT:] * "Two different numbers" means "different numbers in different colors."