Live each day as though it’s your last!
I think it’s supposed to be an upbeat message the idea being to encourage you to have a good time. But the corollary of it is “You’re going to die tomorrow”. If I was going to die tomorrow I’m not sure that having a good time would be that high on my list of things to be thinking about today.
“You only live once”. It’s commonly used as a reason to, say, jump off a cliff, swim in shark infested waters, get drunk and insult the guy who lifts hundreds of pounds, and oter stupid things. But if you really only live once, shouldn’t you try to preserve that “once” as long as possible, instead of possibly ending it doing something dumb?
I’ve heard people say “I’d like to have that nose full of nickels” to describe someone with an oversized schnozz. To me, this is a stupid phrase. You should want it full of *dimes *- you can fit more of them, and they’re worth twice as much.
“You can’t have your cake and eat it too.”
Well, what’s the point of having a cake, then?
I hope I’m not the only one that will write an often used idiomatic expression that I have said for years and think “That can’t be right.” I go to google just to assure myself that I didn’t just make it up or that I am using it correctly.
I’ve adopted YOLODSI as my motto. You only live once, don’t shorten it.
“Stop crying or I’ll give you something to cry about.”
“One of a kind.” If a kind is a group of whatever of a certain sort or type, being just one of a kind isn’t really unique, is it? “Only one of its kind” works better.
The one that bugs me is the old hoary “today is the first day of the rest of your life”. So what does that mean, exactly? Do I have to ignore everything that happened prior to today? Do I have to start over learning to walk and talk and feed myself? Whazzup with that?
Well, once you’ve eaten it, you don’t *have *it anymore.
The expression would make more sense if said," You can’t eat your cake and have it, too." I blame it on our backwards forebears. 
I used to be confused by “the whole nine yards”. Coming at it from an American football viewpoint, it leaves you one yard short.
I’m now told that the expression comes from tailoring.
“I’m just saying”.
Makes no sense and I hate it!
Yeah, we’ve discussed this one before. I think I understand Smapti’s point (there’d be no point in having cake in the first place if you weren’t ever going to eat it), but really, the phrase does make sense to me. Give me a cake, and I’ll be like:
Yay! I have cake!
I want to eat it…
But if I eat it, then I won’t have any cake. 
“All that glitters [or glistens] is not gold”. I once read that it is bad translation from a Latin phrase that actually translates as “Not all that glitters is gold”, which certainly makes sense.
Or worse, ‘know what I’m saying?’ grrrrr :mad:
Or
“you see what I’m saying”.
No I can’t “see” what you’re saying. What am I, on LSD?
It’s perfectly fine when used to mean: "What I just said is not just a statement of fact, it has pragmatic implications. However, I’ll leave you to draw your own conclusions, rather than explain them to you in detail, because 1) I don’t want to insult your intelligence, 2) a lengthy explanation would diminish the pithy elegance of my statement, and 3) a conclusion that you come to yourself is likely to be more persuasive to you than one that is imposed upon you from the outside (i.e. the foundation of Socratic irony as an argumentative technique).
“I think we should steal some of this guy’s money and go to Mexico.”
“He got ten guys in prison killed in the space of two minutes. Just saying.”
However, it is often misused, in statements such as:
“I think your girlfriend is a bitch, and I wouldn’t be at all unhappy if she walked in front of a train. Just saying.”
The first usage is fine, the second maybe not so much. Just saying.
"Fuck you and the horse you rode in on!"
(say what?)
"I’ve gotta go see a man about a horse."
(I use this all the time. My wife says it makes no sense. I say it means I’m going to the bathroom. I picked up this saying in the Marines, although I don’t think it’s a military saying.
My wife still says it makes no sense.)
“For some reason…”
What doesn’t happen for some reason?
No, it’s not a translation of anything, it’s Shakespeare’s formulation of an already well known English phrase (which may ultimately trace back to Aesop, but was definitely in use in English at least 2 centuries before Shakespeare wrote The Merchant of Venice, from which that formulation (albeit with the word ‘glister’) came). And it’s a perfectly sensible, if rather archaic, formulation. If it hadn’t become a stock phrase thanks to Shakespeare, it wouldn’t be phrased that way today, but in the context of Early Modern English, it’s a perfectly uncontroversial formulation.