SHARP DULL

An oxymoron (from Greek ὀξύμωρον, "sharp dull") is a figure of speech that combines contradictory terms. 

Oxymorons are sometimes inadvertently created by errors or sloppiness in conversation.
Examples:   

found missing

 deafening silence

virtual reality  

  pretty ugly   

almost exactly  

exact estimate

seriously funny

tragic comedy

liquid gas

larger half

same difference

constant variable

working holiday

definite maybe

even odds

freezer burn

original copies

jumbo shrimp

living dead

clearly confused

In some cases an inadvertent oxymoron ends up being widely adopted as the name for some concept and ceases to be recognized as an oxymoron. Examples include: bittersweet, living dead, virtual reality.