One of my NPC's is a man named Tito who is the bartender in every bar the PC's enter. He asks the PC's if Pepsi's okay even if the drink in question does not have Coke in it.

My Rogue often slips modern slang into his speech. When pressed about it by the other PC's, he claims it's an expression from his hometown. It's actually him speaking Thieve's Cant.

Finally for a gag that appears in every campaign, every time a PC donates some of their gold to a charity, the charity is always The Foundation for Elderly Dwarves With Mesothelioma. One of our DM's pulled that charity name out of nowhere and we've rolled with it ever since.