One of my favourite linguistic phenomena is the eggcorn. It occurs when a person hears something slightly different to what was said, but the misheard form still makes some sort of sense. If a person never sees the phrase written down, it's very possible for them to think that their interpretation is the correct version.
A good example is the word eggcorn itself, which some people believe is the correct pronunciation and spelling of acorn. It qualifies as a true eggcorn, because acorns are egg-shaped, and corn is a kind of seed, so eggcorn almost makes sense.
A little digging around the internet has revealed some gems, and the Eggcorn Database is a great source. It encourages public submission, discussion, and possible etymologies. It even includes some that it doesn't technically classify as eggcorns.
Here are some of my favourites:
- (chocolate) eclair >> eggclair
- (social) leper >> leopard
- an arm and a leg >> a nominal egg
- eau de cologne >> odor cologne
- cold-hearted >> coal-hearted