I have set a programming challenge to my (university) students. I will also be using this challenge next week when I give a participatory workshop to (school) students. Essentially it is just 2 lines of code with an intentional bug. The code and description of the challenge is at
jsfiddle.net/coas/wda45gLp/
Many years ago when I knew nothing about Computer Science internationalisation and even less about Unicode ie when I was an ASCII programmer, I would have looked at the 2 lines of code and said something like "I cannot see anything wrong with this code"
André Schappo