Curse on the Defense Against the Dark Arts position

The curse on the Defense Against the Darks Arts teaching post at Hogwarts was cast by Voldemort after Dumbledore rejected his offer to teach the subject. Voldemort, in what must have been a fit of pique, basically decided that if he couldn’t have the job, no one else would be able to hold it for more than a year. Since that time in the mid-1960s, each person taking the job left after only one year under varying circumstances. The whole thing was thought by many to be a rumor, however.

“Well, we’ve never had great Defence Against the Dark Arts teachers, have we?” said Harry. “You know what it’s like, Hagrid told us, nobody wants the job; they say it’s jinxed.” (OP13)

“Well, there’s one good thing,” he said savagely. “Snape’ll be gone by the end of the year.”
“What do you mean?” asked Ron.
“That job’s jinxed. No ones lasted more than a year… Quirrell actually died doing it … Personally, I’m going to keep my fingers crossed for another death…” (HBP8)

All the speculation was apparently based in truth. Dumbledore revealed as much to Harry after they viewed the memory of Voldemort’s visit to Hogwarts to ask for the job:

“Oh, he definitely wanted the Defense Against the Dark Arts job,” said Dumbledore. “The aftermath of our little meeting proved that. You see, we have never been able to keep a Defense Against the Dark Arts teacher for longer than a year since I refused the post to Lord Voldemort.” (HBP20)