Malaphor is the term used to describe a blending of idioms or cliches. They can be intentional or unintended, but often create a humourous result, because they tend to occur where the meanings of words or phrases overlap, which can cause a pun-like effect.

A more commonly-used term for them is 'mixed metaphors'.

Some examples:

"I'll double-cross that bridge before I burn it"

"It's not rocket surgery"

"You've opened Pandora's box; now you have to lie in it"