Batman: The Dark Knight
Fans are convinced that when The Joker’s explosions didn’t go off as they should have, he had to improvise. MoviePilot claims that Heath Ledger pressed the trigger button many times but that wasn’t scripted. However they kept it as it fit so well!
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Harry Potter and the Chamber of Secrets
Apparently when Draco Malfoy said his scathing line to side-kick Crabbe, “I didn’t know you could read,” was not in the script! Tom Felton merely forgot his lines.