Whenever they realize they don't understand something. This could happen by reflecting on the material or after making a mistake.
> "Following that tutorial introduces massive security holes into a site."
I've touched on this on other parts of this page.
Which is quite likely to be when their customers' data gets leaked or altered. That's a really bad time to discover a hole in your understanding.
Who would hire someone who only has W3Schools knowledge? Are we really worried about them?
And this pretty much negates your whole argument. If you ever plan on getting hired or being taken seriously enough to get clients, this isn't the resource. How pissed would you be to find out at an interview or after a breach that the site you used to learn all this stuff was the laughing stock of developers?