There are more and more tests out there for inattentional blindness, the phenomenon that every magician knows full well: that when you are paying attention to one thing, other things are invisible. It's called inattentional blindness but it is really "attentional blindness" because it is when you focus your attention in one direction that you can miss something else. This public service ad on YouTube makes the point very well: It's Easy to Miss What You're Not Looking For. It's a lesson that applies to just about every aspect of life. Think about it.
And sorry I spoiled the fun of being tricked and corrected. Now that you've read this, you'll be able to see what you would have missed if you hadn't had your attention directed to it. On the other hand, being aware of inattentional blindness can save you from lots of other errors later on. In fact, the deepest reason for HASTAC is we believe disciplinary focus makes you blind to lots of things and it is only by "collaboration by difference"--working with people who are not so focused on the things that grab you-- that you are able, collectively, to see gorillas (the original experiment was with a gorilla), moonwalking bears, and even the elephant in the room.