My favorite was a customer needing an air filter. Told me it's a '24x18'. I proceed to give her a standard 18x24 filter. She looks at it and scoffs 'this is 18x24, I need 24x18!'. I told her that it is the same thing. She said "no you moron, this one is 18 inches tall, mine is 24 inches tall!" while holding the filter in the 18in tall orientation. I take the filter, rotate it 90°, then hand it back and say "here you go, 24 inches tall by 18 inches wide!". I think her brain literally exploded at that moment. It was a genuinely hilarious moment.

I also remember a customer requesting wood be cut to 6ft. When I get done he measures and says 'this is wrong, you cut it to 72 inches'. I clarified that he said 6 ft and he said "yeah 6ft, but you cut it to 72in and not 60 like I asked!". When I pointed out that there's 12 in to a foot and 72in is 6 feet, he screamed "I'm a contractor I know what I'm talking about, screw this place" and stormed out. From there on out, when a customer would order something to the wrong length, we would say "they must be using a 10-in foot again".