This afternoon, I went to McDonalds for lunch (please no wank about this, I am quite aware that their food is not particularly healthy, etc., but I was in a hurry to get to an appointment, and it was on the way) and got some food items that added up to $8.50. I gave the teenage girl behind the counter a $20 note, then checked my purse quickly and said, "Wait, I've got fifty cents here," and gave it to her. So I paid her exactly twenty dollars and fifty cents.
She gave me eleven dollars in change. I mean, I'm no mathematician, but I'm pretty certain that $20.50 take $8.50 is not $11.00. Actually, I'm pretty certain that it's actually $12.00.
I'm in an OK mood, and hey, everyone makes mistakes, so I smile at the girl and say something along the lines of, "Oh, um, I don't think you gave me enough change, I gave you a twenty-dollar note and fifty cents, so my change should be a dollar more than you gave me." So I give her back the dollar coin, expecting to get two dollars instead.
She looks at me like a deer caught in the headlights and beckons over her fellow cashier, who checks the receipt and nods at her. "No, uh," says the cashier, pointing to the receipt, "you paid twenty dollars, and it cost eight dollars fifty, so your change was eleven dollars fifty." Except that... I didn't get $11.50. I got $11.00.
"No, see," I say, "I also gave you fifty cents, but it's not on the receipt for some reason, so I gave you twenty dollars fifty and my change should be twelve dollars, not- Oh, don't worry, it's no big deal," (as she's still looking extremely lost), and I take back my dollar and go eat.
I wasn't about to get into a huge argument over one dollar, especially seeing as I was probably just going to spend it on junk food anyway (as you do), but really, I didn't think that basic mathematics was that hard.
ETA: The last sentence was a touch bitchy. I'm sorry.