"Honey, 'irregardless' isn't a word."
We're all responsible for occasionally finger-banging the English language, but when you point it out in public, especially around others, you slap a fat social dunce cap on your girlfriend. The consequences will come in three waves: first, she'll be embarrassed and suddenly quiet; next, she'll stew for the rest of the evening, effectively destroying the mood at your gathering; third, your ears will spontaneously start hemorrhaging the moment you get home. By all means, poke good-natured fun at her privately, but if you correct her in public, tell your god to ready for blood.
Flash: women sometimes overreact to stuff. But when something's upsetting her, telling a woman to calm down comes off as dismissive. Want to make things really interesting? Tell her this while you're already fighting. The surest way to ensure she never calms down is to tell her to calm down.
"Do you really need another pair of shoes?"
You think you're being a voice of reason, but the way she sees it, she makes her own money and she can spend it how she wants. If she's awful at managing her finances, and you're worried that your apartment will soon be overrun with leather pumps and strappy sandals, you're probably better off telling her they make her feet look fat.
"We can turn the lights off if you want."
It might seem chivalrous in the moment—she's being coy, you're worried that she's uncomfortable—but don't say it, just do it. She won't think anything of you simply dimming the lights, but if you start providing play-by-play coverage, especially when she's exposed, she'll feel like the under-construction wing of the museum. In this case, words speak louder than actions.
"You're a c---."
In fact, stay away from all four-letter "C" words, including cute, just to be safe. And if she ever does something so atrocious that it warrants the C-bomb, you should say it as you're slamming the door on her.