In Year 1 Marcus Flint is introduced as a Sixth Year student and Slytherin Captain.
(Chapter Eleven, 1st U.S. edition p. 185)
In Year 3 is still mentioned as Slytherin Captain, while he would have finished school by now.
(Chapter Thirteen, 1st U.S. edition p. 263, 305 onwards)
In Year 3, Harry Potter sees someone strangely familiar across the lake who saves many lives.
(Chapter Twenty, 1st U.S. edition p. 385)
Later it is revealed that the familiar someone was not at that spot before travelling back in time. So Harry could not have seen him before the time travel.
(Chapter Twenty-One, 1st U.S. edition p. 411)
[To avoid possible spoilers, this description has been adjusted]
In Year 4, Barty Crouch is referred to as Moody
(1st U.K. edition, p. 594)
[Reported to Bloomsbury by Marleen Palstra]
In Year 4, Cornelius Fudge is referred to as Crouch
(1st U.K. edition, Crouch leaves on p. 485, reference on p. 485)
[Reported to Scholastic by a 9 year old]