“But I booked the field! ...I booked it!”
“Ah ...But I’ve got a specially signed note here from Professor Snape. ‘I, Professor S. Snape, give the Slytherin team permission to practice today on the Quidditch field owing to the need to train their new Seeker’. ”
-- Oliver Wood and Marcus Flint on the Quidditch Pitch (CS7)
Slytherin, 1986 – 1994 (repeated a year)
Quidditch Chaser and captain. Harry thought he looked like he had troll blood. He wasn’t beyond a bit of cheating to win a match (PS11, CS7, CS10, PA13). Flint made Draco Malfoy the new Seeker when Lucius Malfoy donated new Nimbus 2001 broomsticks to the Slytherin Team (CS7).
An honest-to-goodness error in the books has come to be called a "flint," after Marcus Flint, the Slytherin Quidditch captain who JKR inadvertently included in one book too many, making him an "eighth year" student. The term was coined by one of the members of the Harry Potter for Grown Ups list. Remarkably few genuine flints exist, considering the vast scope of the story.
Q: In the first book you said Slytherin house Quidditch captain was sixth year Marcus Flint. If there are only seven years of Hogwarts, why is he in the third book?
Rowling: He had to do a year again! :-) (Sch1)
Quidditch Captain for Slytherin House
"Marcus" - from Latin Marcus, Roman name related to Mars, Roman god of war.
"Flint" - a type of stone that strikes a spark -
Both names seem to be puns concerning Flint's rivalry with Gryffindor Oliver Wood (stone strikes a fire, burns the wood)
Played in the Chamber of Secrets movie by Jamie Yeates IMDB
Yeates wore large, crooked and obviously false teeth to make Flint look more trollish, as Harry described him in the books.