Video Illusions are an amazing way to demonstrate how human brain is imperfect.
Even though you know what there really is, you simply won't be able to really see it, like the first 2 inverted mask illusions.
Watch the following amazing videos and let your eyes be tricked.
Charlie Chaplin Mask Illusion
This hollow face illusion is the best video to start with. Chaplin's mask will turn and although you'll be looking at side which sticks in, your brain will refuse to believe it and you will see it stick out. Try to convince yourself of the opposite, so to see what there really is, and you will most probably fail to see the truth.
Einstein Hollow Face Illusion
Your eyes will be tricked again in this illusion. You can tell your brain what there should be but you simply will see something else either way. Watch the spinning mask very carefully and repeat to yourself what there really should be.
This paper dragon might seem cute at first sight, however, he might look a bit creepy when you realize that he is stalking you wherever you move :-) Of course, dragon's head is concave and that makes for an amazing illusion.
Amazing animated optical illusions.
10 Illusions in 2 Minutes
Classics shown in a fast and funny presentation.
Several good examples compiled.
Longer but fantastic talk that explains how easy we can be tricked.
Another great talk with examples that demonstrate several illusions.