For very young children of a certain age, out of sight is out of mind. If they can't see an object or a person, it's as if it no longer exists in their universe. You can cover their eyes (the "peekaboo" idea) and you soon cease to exist to them.
Then those infants grow up, learn to play chess, and reason like this: "If I play my favorite opening against higher-rated player X, he'll know it better than I do and crush me like a bug. So here's what I'll do [heh heh heh]: I'll play some ridiculous opening I know nothing about, and surprise him!" It's peekaboo all over again. Just because our amateur has decided to engage in a bit of voluntary self-blinding doesn't mean the other player will be clueless against the sideline or joke/gimmick opening. Indeed, one reason why better players are better is that they have more knowledge, so if the weaker player is aware of the sideline, why wouldn't the better player know of it, too? The usual result of this strategy is that the weaker player loses quickly, passes up his best shot to get a good position (your odds are going to be best if you play what you know; so what if he knows it too?), and worst of all misses the chance to learn something. Even if the weaker player loses, he'll get valuable experience by playing his best openings against good players, experience that will pay off against later opposition.
What's even more interesting, I think, is that the peekaboo fallacy is a widespread one intellectually. In the past two days, I've read, and read of, philosophers making pronouncements in areas about which they clearly know very little. Now, it's not that philosophers should remain silent about other disciplines. Their skills can be very useful in evaluating the reasoning and claims made in other fields. They should, however, know what the heck they're talking about before doing so. It was as if their ignorance of the topic implied that no one else had thought or written about it either and had something relevant to say.
So in chess, and in life in general, build on what you know, and not on the assumption of others' ignorance.