Keep seeing the word 'Flint' and wondering what it means?

In our group, 'flint' refers to an actual mistake, accident,
'oops!' made by JKR in the books (as opposed to clues and
red herrings).

It's derived from Marcus Flint, Slytherin Quidditch
Captain/Chaser, stated as being a 'sixth year' student in
the first book, then returning in the third book when he
should have finished his seventh year in book two.

A reader asked JKR about this in an online chat:
http://www.scholastic.com/harrypotter/author/transcript1.htm

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?

JKR:  He had to do a year again! :-)