I hate idioms that don't really make sense (yes, I'm sure they all have their humble etymologies, but I don't care). Here are a couple:
1. "There's more than one way to skin a cat" - What the hell does that mean?! Is there more than one way to skin a cat? How? Are we talking different starting points when inserting the blade that will be doing the skinning? Different skinning patterns, perhaps? This seems like an awfully graphic way to say that there are different ways of doing the same thing, don't you think? I don't even want to know one way to skin a cat, let alone more than one.
2. "You can't have your cake and eat it too" - This one's pissed me off for years. What the fuck good is my cake if I can't eat it?! (Curmudgeon's note: the one exception to this is if you get one of those fancy cakes that they make on Ace of Cakes on the Food Network. Those are awesome and probably worth holding on to.) The whole point of having a cake is eating it. Everyone knows that. How about this: how about you have the cake, and I'll just eat it? Sound good? You have, I'll eat. Stupidest expression ever.
There are tons of good ones (and by good, I mean bad) out there, but these are the two that most immediately occurred to me. Feel free to suggest other ones for me to bitch about.