There are several types of mix up with words that have amusing results.
For example. Using the wrong word in a saying: my friend's mum is always saying things like 'going off on a tandem'
instead of a tangent or 'putting someone on a pedal stool'
My favourite is mixing idioms.
I'll burn that bridge when I come to it. Or,
You've opened a can of worms, now lie in it!'
My absolute favourite:
'Til the cows freeze over