I work at parts store for lawnmowers and one day we had a guy come in looking for a belt for his 46″ cut Murray. Since there can literally be dozens of varieties I as ked for his particular model number. The instant deer in headlights look said no model number. Miraculously I found the correct belt but we had to order it. Disgusted he looked up and saw dozens of belts hanging from the wall and asked if the numbers on them told what deck size they fit. I honestly answered yes along with the part number they also some times include information pertaining to the deck they are intended for. So he smugly pointed out I had a Murray branded belt that said it would fit a 46″ deck and how “he didn’t need no computer neither and something about parts people these days being lazy and all murray 46″ decks are all the same” so while I am imagining him falling into an active volcano I retrieved the belt and lo and behold it was about a foot too long as he compared it to the old belt he already had he asked me as if it was my fault “how come this is different” when I explained that Murray used about six different decks in his size he made me check to see if the right belt was in the right sleeve when he left without the belt without ordering the right belt and more confused than ever it took all I had not to ask if he learned anything

(I am about to move and ran out of boxes. I head to the nearest hardware store to buy two. It’s Wednesday morning, so the shop is dead; I am the only customer. I get my boxes and head to the checkout. The boxes get scanned by a bored cashier, for 5€ in total. I pay with a 20€.)

Cashier: “Here is your change.”

(She hands me a 5€ note.)

Me: “But I paid with a 20€.”

Cashier: “No, you didn’t. You paid with a 10€ note.”

Me: “I am sure I did pay with a 20€ note!”

Cashier: “Yeah, but to check I would have to count all my change. This would take a while.”

(She is obviously not interested in doing so.)

Me: “Please do. I have time.”

(She sighs heavily and calls for a second cashier. Without even looking at me, both start counting the money, which takes twenty minutes. In the end, it becomes obvious that she has 10€ too much in her till. Without further comment or even an apology she hands me two 5€ notes.)

Me: “Can you please give me a 10€?”

(She looks into her till and shakes her head.)

Cashier: “I don’t have a 10€ note in my till.”

Me: “…”

(The customer in question asks to look at a pack of sandpaper. After fetching it I note that it explicitly says ‘hand sandpaper’)
Customer: No, no, the machine would pull this stuff apart.
Me: Sir, might I suggest finding the correct sanding sheets for your machine? We have a lot of different ones.
Customer: No, No, I need a good strong one that I can cut down to size.
Me: Sir, the box says ‘hand sandpaper’ that means that it’s not designed to take the stress that would be forced on it by a sanding machine. It’s designed to be used by hand.
(The customer completely ignores me, repeats himself, and then talks about trying to find ‘better sandpaper’ I had to fight not the facepalm before finishing the order, but fortunately, I was able to get away from the tills and plant my head on the manager’s desk, out of sight of the tills.)
Manager: Is everything all right?
Me: I thought that it was difficult to be dumb in the building trade, someone proved me wrong.

(We are primarily a wholesaler to builders and similar trades. The customer is complaining about a draught in his home and asks to look at some insulation, which looks like silver bubble wrap, at which point he kicks off, complaining about his home being cold, but what drives me over the edge)
Customer: I know what you are trying to pull here, this is just a gimmick!
Me: Sir! I’m fairly new to this job, but I can tell you that this insulation is based on the same principles as a thermal mug. There are three ways heat energy transfers, one is conduction, and this requires a solid. This is why there are bubbles of air in this insulation. Second is convection, which passes through liquids and gases, that is why the bubbles are small. Finally, there is radiation, which does not require either, however, heat transfer in this format is affected by the colour of the material in question. Silver is the worst conductor of heat radiation.
Customer: … *gets a paper and pen* Can you repeat that all for me? I really need to write this down.
(the sad part, I learned this during my GCSEs, and I’m in my late twenties now.)

(We have a guy who is known for being surly to colleagues, and the only female he talks to is a girl of similar origin to his own, this happens in front of him, I’m a white female)
Me: Collection number [number]!
Customer: That’ll be me love, how are you today?
Me: Doing very well sir, and yourself?
Customer: Not bad, not bad, talking to [coworker] here waiting for my order. I hear people say good things about him, me, I just tell the truth.
(A few weeks prior, this guy demeaned the team I’m in to a customer, while right next to me. Thank you sir, for being a great regular and dishing a hot dose of karma!)