There are only two hard things in Computer Science: cache invalidation, naming things, and off-by-one errors.
Now I'm going to have to link to one of my favourite sources of engineering wisdom, The Codeless Code (which deserves to be way better known than it is).
Or intellectual property protection. Their whole concept is hiding the message in the noise. (paranoid voice) "Don't read the April-related stuff! Highlight the page to see the hidden comments! The real reason they created it!"
You haven't heard of and?
Ha! That's funny, noone has noticed that before you :) I wonder if I was thinking of a fifth or not...