
Continuity mistake: After Harry has caught the Snitch and is sitting up, Hermione runs over to him and kneels in the sand next to him. Her robes are covering her knees in a long shot, but as it cuts to a close up her knees can be seen, and this continues to switch for the next couple of shots. (00:57:55)
Answer: Yes. It's a common occurrence in a lot of movies for actors to forget their original lines so they improvise something to help move the scene along and not break character. Tom did this because he forgot what his original line was.