I've never understood the phrase: "You're comparing apples to oranges." It's supposed to mean that you're trying to compare two uncomparible things. But they're both fruits of course you can compare them!
I've also always had a hard time with: "You've got your work cut out for you." Supposedly that means that your upcoming work will be very difficult. To me, though, it sounds like the work is mostly done already. If something has been cut out for me, that means that I don't have to cut it out myself.