Idiomatic expressions are one of the hardest aspects of learning a second language because what they say is not necessarily what they mean!
Lets see some examples...
"I could care less." ~> means that you don't care BUT if you "care less" then at least you care some!
"Like chalk and cheese"~> means not alike but you can't substitute a chalk for a cheese right?
"To overlook" ~> means to miss something you should have noticed but if you are over-doing something, isn't that doing it too much?
" That's a horse of another color" ~> means another matter or subject altogether not necessarily related to animals
Do you know of any other expressions? I know that after so many years, I still struggle with "the alarm going off" in the morning so Ring along!