This is a fun thread, and I want to join the fun.
1. Twins. My two youngest were a 'Buy One Get Another Full Price' Deal. (Identical.) They were once asked, "You two are twins, right? Are you just twins, or are you also sisters?"
2. Geography. My credit card was once declined at a grocery in Omaha, Nebraska. I asked the cashier to suspend the sale and serve the customer next in line while I called the number on the back. The credit card rep alerted me to fraudulent activity on my card! I live in Iowa! Someone was using my card in Nebraska! I asked, "Do you how close Iowa and Nebraska are to one another?" "No, how close are they?" Probably not my shiniest eHell moment, but I responded, "There are parts of Iowa that are IN Nebraska. It's called Carter Lake, Iowa. The river shifted after the state lines were drawn." She restored the use of my card.
3. Percentages & Decimals. Joking but dead-panning, I once told the principal at my childrens' elementary school, "I just read a study that claims that 50% of school administrators graduated in the bottom half of their class." He was horrified. (I was, in turn, horrified.)
4. Dumb Signs. Once in a motel pool, along with "Don't dive in shallow end" and other random disclaimers was a new-to-us warning. "Please do not blow your nose in the pool." My kids were aghast and asked why the sign was there. I said, "Well, apparently it has happened and it annoys Management."