I teach CPR among other things. My favorites are the ones that walk into class, see 10 manikins on the tables with a book on top of each that says CPR very prominently on the cover. They look at me sitting behind yet another manikin on the desk, a CPR mask, a defibrillator and gloves displayed in all their glory. Then, scan the room once more and proceed to ask me, "Is this the CPR class?"

I usually just say, "No" and then wait. Most will smile sheepishly and come in and a few will say, "Right, ok I guess I deserved that." It's a good thing I'm easily amused. One of these days I'm just going to say that it's underwater basket weaving class and did they remember to bring their dive mask.
I had one person call me and ask if I gave free CPR classes. I said, "no." There was a beat of dead silence on the phone, then we both just laughed and laughed.
I'm lucky though in this job. My customers/students are all pretty awesome and very pleasant. It is extremely rare that I get someone who is not nice.
Years ago when I was in college waiting tables I did get one memorable customer. I was 20 years old and it was the day before my birthday so I had my hair done in pig tails to celebrate my last day of of being "underage". I went to take the order for a group of four men and when I got to the last guy, he looked me up and down and said, "I think I"ll have a fur burger." I was stunned into silence. Then he quickly gave me his real order before I could react. I went into the back to get ring in their order. I was so mad and wanted to get back at the guy. I grabbed a plate some burger buns and opened them up, placed lettuce and tomato on one of the buns. Then I cut off half of one of my pigtails and placed it on the other bun. I stormed back into the dining room, slammed the plate on the table in front of the guy and said, "Here's your fur burger. Do you want ketchup with that?" His buddies burst out laughing and he was now stunned into silence. Then he started cracking up. They wound up leaving me a really good tip and became regulars that would ask for me every time they came in to eat. LOL