Care to elucidate?

It's a fairly subtle way to dramatically change the behavior of your code. Frankly, it's a timebomb threatening to run testCode() in your production site.

The low cost makes it useful for quick development, sure, but for stability's and sanity's sake, please remove when you start promoting this above your own playground.

I think the poster might be referring to using finicky, unreadable line noise. Expressing program logic this was will inevitably lead to misunderstandings.