In this short story, there is a time travel agency which allows people to go back in time to mentor or encourage historical or famous people in their pursuits.
The main character visits his favorite sci-fi author, but he is so inept that he changes history: the author writes a scathing essay about sci-fi and time travel being inane topics, and he never becomes an author.
The agency, annoyed by the man's failure, tell him they will be sending him back to mentor Hitler with the idea being he will do such a terrible job that Hitler will never rise to power.