I bought two twelve packs of soda for my boyfriend/his roommate. I spend a few nights a week and don't help with rent, it's the least I can do. I'm checking out and not paying attention to what she's doing, as I'm doing the whole debit card machine thing and trying not to pass out for various reasons. She says, "You know, the sodas are on sale, buy two get one free." I look up, smile, and say politely, "I know," and then get back to my PIN. I finish paying, and am almost out the door, when she yells, "Ma'am! Don't forget your soda!" I thought I had forgotten one at the checkstand, but I look in my cart and they're both there. She had charged me for another one. Rather than get a refund, which is an absolute pain with this sale (I worked at a different store, same chain, for two years), I got to go and get another soda. It just pissed me off because A) they didn't have the exact one I wanted in the first place (I had to get regular sprite instead of sprite zero, ugh.) and B) it's not actually free, you still have to pay CRV.
Long story short: Ask before adding another "free" item to my bill, kthx.