Case like that are straightforward, and people still manage to botch them. Which is why I suspect there is something deeply wrong, or at least fundamentally different, about the minds that produce such mistakes.

Though for this particular mistake, it may have come from some misguided notion about performance (like perceived overhead of constructing a string then passing it up the stack).

As for things like recursion and closures… I just avoid them because I know my colleagues will recoil in horror with such uselessly indecipherable excess of cleverness.