Oh no wait, I thought of another one. This happened when I was working in a bakery a few months ago:
I wasn't really looking directly at the person who was looking through the bulk bagle/bread bins we had, I just noticed long hair out the corner of my eye cause I was busy packaging other things, and I say, "Can I help you find anything today ma'am?" and at that moment, the customer looks up and its a man, mustache and all and I just go ". . .I meant Sir, omg I am so sorry."