Okay, I'll show you a series of numbers. Add them up, one after the other. But do it in your head, not on paper or with a calculator. Add the numbers as you scroll down, and try not to dawdle. Saying the answers out loud may also help

Lets see if your mind is tricked as easily as mine was. (sorry for the vertical stretch mods, but it's necessary.)
1000
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>40
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>10
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>1000
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>20
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>1000
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>20
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>1000
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>10
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Now.... You should have added all these numbers up together.
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>Think of you answer. Keep it in your head
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Is your answer 5000, or 4100? If it's 5000, you're wrong

The correct answer is in fact, 4100, but most people end up thinking that the answer is 5000. It's a strange thing, and I forgot why it happened, but hey, there you go.