Ok, there are several examples of this, I don't doubt you will all enjoy trying to find new ones, but one thing I've never understood.
To me if you preceded a word with in/un/de it kind of flips the word.
inconspicuous vs conspicuous
discrete vs indiscreet
So why do some words not have that?

Define....
in·gen·ious - Clever, original, and inventive
gen·ius - Exceptional intellectual or creative power or other natural ability
There are so many examples where the un/in/de word means the same, any many where it doesn't.
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