How we change what others think, feel, believe and do
A mixed metaphor is one where the metaphor is internally inconsistent, for example where multiple metaphors are used which do not align with one another.
The metaphors used often have some connection, although this is often tenuous or inappropriate.
He's a loose cannon who always goes off the deep end.
Footloose and fancy-pants.
He often shot his mouth off in the dark.
A rolling stone gathers no bird in the hand.
It was playing with fire in the belly.
Mixed metaphors are typically a result of trying to be too elaborate in speech and perhaps careless in the selection of metaphor.
The result can be quite comic. This gives opportunity to use humor for deliberate effect.