ForceSlytherin

Question: Why is Hermione wearing a bandage? I see no injury to her hand but when they travel back in time she has a bandage on her hand.

ForceSlytherin

Answer: You can see blood on Hermione's hand when Ron says "Scabbers has been in my family for..." before Sirius says "12 years"

Chosen answer: There's no reason given and it is not part of the story line. Most likely, at some point during the filming of this scene, Emma Watson suffered some minor injury and needed to wear a bandage.

raywest

Answer: She could have injured it while she was on the Whomping Willow.