Audio problem: When Ron and Harry are walking down the corridor towards Potions class, Ron says, "...Quidditch trials coming up, I need to practice," but his mouth is not moving and/or not in sync with that dialogue.
Audio problem: When the trio are on the Hogwarts Express, we hear Harry say "What was Draco doing with that weird looking cabinet and who were all those people" but the subtitles say it was Ron who said that.
Chosen answer: Snape wants to know how Harry knew Sectum Sempra. As shown by him using Occumelancy to see where his copy of Advanced Potion making is. But at the same time, if Snape punished Harry for a spell in a book, Harry could show it to Dumbledore, who would recognise Snape's handwriting. Additionally, Harry was certainly punished in the book - he got detention with Snape every Saturday for the rest of the term. The movie most likely had to cut this for time issues, and also perhaps because the scene is more dramatic if Snape only silently glares at him.
Ssiscool ★