...by just met I mean within about 24 hours.
1. "My girlfriend/my boyfriend" more than 3 times. You then look like the douchebag that feels the need to talk about their significant other incessantly. Odds are, the impression that the person you're talking to is going to think you're a weirdo who has to keep saying "boy/girlfriend" over and over because you can't believe you actually scored one.
EXCEPTION: If they bring it up first its ok, because at that point you're just working on-par with the game they've already laid.
2. Your reasons for dietary restrictions. Telling someone you just met that you can't eat seafood because if gives you the shits is too much of an over share.
EXCEPTION: its an allergy that will kill you.
3. Your medical history. I don't need to know that you wound up in the hospital in Thailand because you cut your foot in the jungle (wearing flip flops) and it got infected. Also don't need to know about the mucusy cold you just got over.
EXCEPTION: It's relevant to the current situation. Suitbale alternatives:: "Sorry, I can't go hiking; I hurt my foot." Or maybe, "I can go out for a little while, but I've been sick recently."
4. Randomly bring up some huge personal/family tragedy. I don't know you well enough to know how to react properly to "My grandmother died yesterday" or "I had family killed on 9/11." Not that you should feel like you need to keep it locked up, but just wait until we've known each other a little better before you tell me something that sad.
EXCEPTION: You're at a monument/memorial dedicated to the tragedy. But even then, proceed with caution.
5. "Whoaaa (about someone nearby/passerby)~ he/she is HOT!" You've just successfully ruined your chances with whomever you're talking to. Even if you're not actually interested in the person you're chatting with, maybe they're secretly slutty and you just totally ruined your chances of hooking up.
EXCEPTION: Its someone famous. And by all means, look all you want; just keep your mouth shut.