Half Blood Prince's Identity
Harry finds an old potions book which is "Property of the Half-Blood Prince." Throughout the book, Harry is greatly helped by the notes written in the margins of this textbook, and begins to feel like he knows the Half-Blood Prince. He even considers him a friend. Then, at the end of the novel, it is revealed that the Half-Blood Prince is Harry's enemy, Professor Snape. So, Harry has unknowingly spent his entire school year becoming excellent at Potions thanks to the teacher he despises the most.
Ron's Poisoning
After Slughorn cures Ron of his love potion, he offers him some matured mead to celebrate. This mead, however, is poisoned, which makes Ron much sicker than he was before. This is also ironic because Slughorn intended to give this mead to Dumbledore, which would have most likely killed him if he did. However, he forgot, and inadvertently saved the headmaster's life.
The Fake Locket
Harry and Dumbledore go on a mission to obtain a Horcrux, which is a piece of Voldemort's soul. They successfully get this Horcrux, but the mission saps Dumbledore's strength and makes him unable to fight back when Snape fights and ultimately kills him. However, all of this is pointless, because the Horcrux turns out to be a fake. So, Dumbledore gave his life for nothing.
The Diadem
When he goes to hide the Half-Blood Prince's book in the Room of Requirement, Harry marks his hiding place with what he thinks is just a regular tiara. Only later does he realize that this was a Horcrux, the very things he and Dumbledore risk their lives trying to find. It could have easily been destroyed then, but Harry simply did not recognize it for what it was.
The Unbreakable Vow
In the beginning of the novel, Snape gives Narcissa, Draco's mother, an unbreakable vow that he would protect Draco at all costs, and kill Dumbledore for him if Draco could not. This is ironic because Snape already knew that Draco had to kill Dumbledore, and knew Draco would ultimately fail. Snape had already planned to kill Dumbledore anyway. So, it was unnecessary to make this vow.
Dumbledore's Death
Dumbledore had spent his entire life promising Harry that Snape was good, and on their side. He tells Harry to do nothing when Snape enters the tower where Dumbledore stood in his weakened state. However, Snape murders Dumbledore, the man who in life had constantly defended Snape's nature.
