My first one (not medical related) was when I was much younger, and a waitress at a Coney Island restaurant. We had a ton of regulars and it was very common for them to come in at lunch and order a couple Coney dogs and a Coke. I had one male customer who ordered his two coneys but ordered a root beer instead. After delivering his food and drink, he said "Excuse me miss but I don't think this is root beer" to which I replied, "Oh no, did I grab your C*** (another word for rooster) instead??" I instantly turned red, and said "I meant Coke!" He laughed, I then hid, mortified until he left. He must've felt pretty bad for me though, he left a nice tip.
My second one actually happened to someone else, but I was the witness who cringed. While working in the ER, we had a young girl brought in after a drowning incident in which ROSC was achieved and she was somewhat alert. After everything was settled, the xray tech came in to take some films, and told the girl "Okay, take a deep breath and hold it like you're going under water!"