I've got a two-way tie for 'dumbest thing ever said'.
#1) I was telling a story about a friend of mine, where he relayed to me about the dumbest thing HE ever said. He was sitting on a cliff on the California coast overlooking the ocean with another friend of mine, getting a good buzz on, looking at the sunset, and remarked, "We should sit here all night, and watch the sunrise. It'd be awesome coming over the ocean."
Future Mrs Proceed then laughed and remarked, "HA! Cuz the sun doesn't rise at night, right?"
#2) I was going to try to expand my lady's pallet by making her a grilled teriyaki tuna steak with wasabi sauce. She said, "Gross. Why would you grill tuna in a steak?"
That's not the dumb part. I knew what she meant, she meant 'why would you form shredded tuna fish (like in a can) into a steak-like shape and grill it?'
After some talk, she finally said, "But how do they get a steak from all those different kinds of fish? Is it like a bunch of big fish pieces?"
I asked her, "Eh?" She said, "You know, Tuna is just all the garbage fish nobody wants, all chopped up." I told her that Tuna was actually a species of fish, a fairly large one, and that actual tuna-steaks are expensive, and pretty darn awesome.
She flatly said, "Are you sure?"