"What did you do to your hair?"
Unless we've cut our own hair—this is not common—someone else did something to our hair. It wasn't us. And most likely we've gone to a lot of trouble and expense for it. "I like your new haircut" is infinitely better, and shows you're paying attention. It's also far superior to the generic "You look different," which tells us you're as clueless as ever.
"They both look the same to me."
We understand you care a lot less than we do about the outfits or the registry dishware we're asking you to compare. But they can't possibly look exactly the same, can they? Give us something. Anything. Mentally roll the dice and pick one, so we don't worry about your vision—or worse, that you don't care.
A kissing cousin to "Don't get so worked up," this generally creates the exact opposite effect you're shooting for. When you say "Relax," what we hear is that you think that we're being irrational over nothing, and this makes us do anything but relax.
"I've got it all under control."
Ha! Famous last words. Refrain from using them if you don't want us to take fiendish delight in your getting lost because you won't stop for directions (if we're late, there will be fiendish fuming), or because you're missing a piece to your flat-screen television because you said you didn't need to read the assembly instructions.
"You're not one of those feminists, are you?"
Yikes. Chivalry may be nearly dead, but saying this will drive the last spear through its heart. Feminist or not, a woman is likely to be offended by the question. Just be yourself. Be kind, open the door, offer to pay, and go from there. We can choose to accept or share in your generosity.
"When are you due?"
Take one second to imagine a woman turning to you and responding, "I'm not pregnant," or "I had the baby six months ago," and you'll understand why you should eradicate this question from your vocabulary. In one nanosecond, innocent—even considerate—curiosity can turn to deadly, if unintentional, offense. And there's just no way to recover from this one.
"You're being too emotional."
In the heat of the moment this may be true. But unless you want your partner to become more emotional or get angry, you're better off keeping this observation and its off-limits follow-up question—"Is it that time of month?"—to yourself.
"You're acting just like your mother, my mother, or my ex-girlfriend."
All three are problematic. An ex should be mentioned sparingly, and never in comparison. Why would we want to remind you of a person you broke up with? And come to mention it, why are you thinking about her? You see the slippery slope. Conjuring an image of our mother or your mother can be equally grating. We want you to treat us as individuals and not as mere products of your (or our) upbringing.
"You talk too much"
We've seen romantic comedies, and while we may (or may not) like cheesy movie lines, they usually fail in real life. We understand that the possibility of romance makes inexplicable things come out of a man's—and sometimes a woman's—mouth, but please listen and keep the compliments real, honest and sincere and for God's sake... Say you love her only when you mean it.
"You are getting fat... Do you really think you should be eating that?"
Say yes and agree. She should be eating it. Even if she told you she has given it up.