"Because I thought--and Professor McGonagall agrees with me--that that broom was probably sent to Harry by Sirius Black!"
--Hermione Granger (PA11)
Harry wakes up to a pile of presents at the foot of his bed. Among them is a long thin box with no note attached. When Harry opens it a brand new Firebolt rolls out of the wrappings. Harry and Ron are in shock. They discuss who could have possibly sent this expensive gift to him, Dumbledore and Lupin are Ron’s suggestions, but Harry quickly dismisses both ideas. When they tell Hermione about the broom she has her own idea of who sent the anonymous gift. She shares her theory with Professor McGonagall after Christmas dinner, who then comes to the Gryffindor Tower to confiscate the broom. Harry and Ron are furious with Hermione. She however did what she thought was best for Harry, because Hermione believes that Sirius Black sent that broom to Harry to cause him harm.
Hermione was correct in thinking that Sirius Black sent that broomstick to Harry. She was just incorrect about his motives.