Search "328" and the next XKCD is no-where to be seen.
The lesson, according to this character and so many real people on the internet, is to sanitize your inputs. The school in the cartoon didn't sanitize its inputs - and one of its database tables got deleted!
Ask anyone about developing websites and they will tell you the first lesson is always to sanitize your inputs. In this day and age you'd have to be crazy not to sanitize your inputs.
Trouble is, sanitizing your inputs is very bad advice.