Givens,
It just struck me the other night that some people can go through their whole life with a certifiable disorder / condition and never know. If the sky looks pink to someone with colour blindness but they've been told the colour is 'blue' and to them it's normal, how will they ever know unless they take a test?
And even then, who is to say that the person with 'colour blindness' doesn't have the correct vision and everyone else has an altered view.
Test for colour blindness.
There's probably a huge list of things like this, if anyone knows / can think of any more please post up.