If you’ve ever seen an episode of the TV show Kath & Kim then you’ve heard at least one ‘malapropism’, where a similar sounding word or phrase is used mistakenly for the appropriate word or phrase making it nonsensical:
“I want to be effluent mum!” – from Kath & Kim
This is funny both because she means she wants to be affluent, and because effluent in this context means almost the opposite to what she intends. We call them malaprop-isms after a character named Mrs Malaprop (from a play called The Rivals) who uses plenty of them.
Obviously malapropisms are used deliberately for comic effect, but you come across real people using them. I had a colleague who was famous for them, and I always felt a bit sorry for him because he was so unaware he was doing it.