Here's one that I found really interesting:

There is no reliable scientific evidence that installing "security lighting" in outdoor areas actually deters crime; it may actually make crime easier. For instance, a burglar who is forced to use a flashlight is more easily spotted than one who can see by existing light.
Source(s):
http://www.wikipedia.com

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The actual words to any Dylan song..
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the misconception that UFOs don't exist.

of course they exist they are just "unidentified" flying objects.
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"If you go outside dressed like *that* in *this* weather, you'll catch your death of cold!"

Being cold does not cause colds.
Source(s):
Mothers everywhere, and http://www3.niaid.nih.gov/topics/commonCold/cause.htm

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That people need to drink eight glasses of water a day and the companion belief that if you feel thirsty, you're already dehydrated. It doesn't have to be water, it doesn't have to be 8 glasses and a good time to drink is when you're thirsty.
Source(s):
http://www.snopes.com/medical/myths/8glasses.asp

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