Dobby blocks the barrier at King’s Cross

From a hiding spot where he has been watching for Harry, Dobby blocks the gateway to Platform 9 3/4 when Harry and Ron try to go through it, causing them to miss the Hogwarts Express (CS5, CS10).

Timeline Notes

The rest of the Weasleys make it through the barrier with only minutes to spare before the Hogwarts Express's 11:00 departure. Bringing up the rear, Harry and Ron crash into the barrier less than a minute before 11:00 (CS5).

Harry does not find out that Dobby was the cause of this obstacle until they meet in the Hospital Wing on the night of November seventh (after Harry was injured by Dobby's bludger) (CS10).