Arthur Weasley standing up to Dumbledore and telling him, no, Harry will not be taken back to the Dursleys, not under any circumstances.
In chapter 33, Neville Longbottom standing up to his grandmother by sending her a Howler. Everybody within hearing range is shocked that anybody could send a Howler to Augusta Longbottom. And then one of the ladies realizes that the Howler is a result of her own son's gossiping.
Also, in the same chapter, Harry silencing Augusta's own Howler (the direct cause of Neville's Howler) and Neville's explaining to McGonagall his grandmother had sent it because Neville had "offended her bigoted, pure-blood sensibilities". And the group giving the gossiper the deserved punishment: even Hermione joined in, if only by teaching Harry a curse to make him look like an idiot.
And about that punishment... Harry tried to assure McGonagall everything was under control, nobody would do anything and smile reassuringly, but the teacher was terrified.