"The Thumbprint of Doom" is a short story in Even More Tales to Give You Goosebumps.
Trisha thought she'd have a great summer, but it's been hampered by her friend Jeremy's cousin Harold, who is more interested in reading than having fun. Harold doesn't even want to go swimming, as he'd rather stay home and read the dictionary.
Luckily, a new girl has moved onto the street. Her name is Carla and she follows horoscopes and other new age superstitions very closely. She tells her new friends that the thing she fears most of all is the Thumbprint of Doom. It's an ancient spell that curses the person who receives it with death within 24 hours.
Unluckily for Carla's new friends, Carla takes her superstitions very seriously. She won't even let Trisha ride in a blue canoe on a Tuesday. Trisha and Jeremy talk both Carla and Harold into visiting the local fair. Once on the grounds, they get dragged to a fortune teller who reads Carla's fortune... and then in fear plants the Thumbprint of Doom on her forehead, to "spare" her the horrors she foresaw. Carla sprints out of the tent in terror.
Trisha and Jeremy delight in their joke. They snuck away to the fairgrounds that morning and paid the fortune teller to pull the stunt as a joke. They go out to tell Carla about the gag. Carla snaps at them and reveals that she knew it was a gag, as she's the only one who can give out the Thumbprint of Doom. But now that they know that, she'll have to imprint it on all three of them. She does so and the kids run off into the night, screaming.
The fortune teller sticks her head out of her tent and asks her daughter, Carla, how long it'll take her friends to realize that Carla didn't actually have any powers and was playing a trick on them.