FlatulentFish All Kinds of Junk

Read this in the Annotated Pratchett File, specifically in the section about The Color of Magic; it is regarding the "well-known folktale" Appointment in Samarra:

For those who aren't familiar with the story, it concerns a servant to a rich Baghdad merchant who goes to the market and encounters Death there, who gestures at him. Convinced that this is a very bad omen indeed, the servant rushes back to his master in a great panic and begs him for a horse, so that he can ride to Samarra and escape whatever calamity will befall him should he stay in Baghdad. The kind master gives the servant a horse, and goes out to investigate for himself. When the merchant finds Death and asks him why he frightened the servant so, Death replies: "I wasn't trying to scare him, it is just that I was so very surprised to meet him here, because I have an appointment with him tonight in Samarra!"

And so it goes...