"Malfoy must've been inside the Room at the time, so she - what am I talking about? - he dropped the scales to tell Malfoy not to come out, because there was someone there!" -- Harry Potter (HBP21)
Draco gets Crabbe and Goyle to carry scales when they are look outs for him outside the Room of Requirement. They drop the scales whenever someone comes by the Barnabas the Barmy tapestry in order to make noise and warn Draco someone is near (HBP20).
"She looked terrified at the sight of the approaching sixth-years and dropped the heavy brass scales she was carrying. 'It's all right!' said Hermione kindly, hurrying forwards to help her. 'Here...' She tapped the broken scales with her wand and said, 'Reparo'" (HBP20)