Here are two misuse of language which bother me more than they should.
The first is "The proof is in the pudding." NO, no, no. The line is "The proof of the pudding is in the tasting" and it's degenerated into the proof is in the pudding and it annoys me every time.
Believe it or not, I heard this one twice today: It's a doggy dog world. No, no, no. It's a dog eat dog world.