Somewhat related, I once sent a FOI request to a government agency that decided the most secure way to redact documents was to print them, use a permanent marker, and then scan them. Unfortunately they used dye based markers over laser print, so simply throwing the document into Photoshop and turning up the contrast made it readable.

I remember noticing that a teacher in high school had used white-out to hide the marks for the correct multiple choice answer on final exam practice questions before copying them. Then she literally cut-and-pasted questions from the practice questions for the final. I did mediocre on the essay, but got the highest score in the class on the multiple choice questions, because I could see little black dots where the white out was used.