Uncle Sirius's Big Mistake

Uncle Sirius's Big Mistake is the second chapter in Harry Potter and the Prisoner of Azkaban.

Summary
Harry is staying at Sirius's Muggle flat and he is not happy that people are still keeping secrets from him. After Sirius dozes off, Harry occupies his time looking through the old notebook Sirius gave him last year. After making some tea and toast, he finds Sirius awake and, what appears to be the first time, looking through the letters of the notebook. Their attention is drawn to the television, where a Muggle reporter is notifying the public of an escaped convict, who bares some similarities to Sirius. A few days later, Nymphadora Tonks visits Sirius and Harry eavesdrops on their conversation about their concerns with Regulus Black on the loose. A few more days pass and Harry is still stuck with Sirius in his flat. Remus visits them, and tells Harry he is going to be Harry's new professor for Defense Against the Dark Arts at Hogwarts.At Remus's urging, Sirius tells Harry that his brother was a well known supporter of Voldemort and was sent to Azkaban, and has broken out, leading some to worry he might start hunting Voldemort's enemies out of loyalty and vengeance. Harry is not completely convinced Regulus is guilty, because of the letters he read from the notebook, implying Regulus was leaving the Death Eaters. Sirius is angered that Harry might be starting to sympathize with Death Eaters, so he takes the notebook and the letters in it and burns them. Furious, Harry runs away.

Deviation from Canon

 * The canon version of the chapter is called Aunt Marge's Big Mistake (Aunt Marge being an aunt from the Dursleys)
 * In the chapter, Marge continues to insult Harry and his deceased parents, and Harry's anger produces magic to inflate Marge like a balloon, and he runs away.
 * Harry did not know of Azkaban by this point in canon.