in past Word Ways articles, I've differentiated between true mixed metaphors, which combine incongruous, absurd, or unreal images ("it's the whole kettle of fish in a nutshell") and what I call mashed metaphors -where clashing tropes and cliches aren't contradictory or impossible, but nevertheless create an awkward and ludicrous effect because of their proximity ("Investors smelling trouble can vote with their feet").
"Opening the Floodgates to a Pandora's Box of Worms,"
Word Ways: Vol. 46
, Article 9.
Available at: http://digitalcommons.butler.edu/wordways/vol46/iss4/9