One of the most gut-wrenching deaths in the Harry Potter series is that of Sirius Black. It was also a death that only happened as a result of a plot hole due to an event earlier in the same novel, Harry Potter and the Order of the Phoenix.
Before he returned to Hogwarts after the Christmas break, Sirius gave Harry a badly wrapped package and told him to use its contents to contact him should Harry ever need to talk. However, Harry vowed never to open the present because he didn't want to do anything that might cause his godfather to leave the safety of 12 Grimmauld Place.
By never opening the gift, Harry apparently entirely forgot the very last conversation he had with his godfather before returning to school. This seems like a stretch, but it is not necessarily unbelievable because Harry had an awful lot going on during his fifth year. Between his complicated relationship with Cho, Umbridge being hellbent on ruining his life and trying to learn occlumency, he was pretty preoccupied.
Yet, forgetting Sirius' gift was a communication device led Harry to break into Umbridge's office to use her unmonitored fireplace to access the floo network so he could speak to Sirius. What seems like an inexplicable plot hole is that Sirius seems to have forgotten all about his gift to Harry as well.
When Harry arrives in the fire, Sirius never even questions why Harry has decided to use the much riskier floo network instead of the very convenient two-way magic mirror Sirius had given him just a couple of months earlier. Sirius would have known exactly what was in the package he gave Harry, and therefore, should have been surprised not only by Harry's sudden appearance in his fireplace, but by Harry's decision not to use the tool he was given for this exact scenario.
Sure, maybe his shock at seeing Harry drove the existence of the mirror right from Sirius' mind. And yes, their fire conversation was cut short by the appearance of Argus Filch. However, Sirius would know better. Sirius would have mentioned the mirror. The only reason he did not is because it better served the plot and allowed Harry to fail to contact him prior to his trip to the Department of Mysteries. This gaping omission is not in keeping with Sirius' previous behavior, and feels notable because there is no explanation for him neglecting to mention the mirror that does not feel contrived.
In fact, it is Lupin who is sitting in front of the fireplace when Harry appears suddenly in the fire, so it is Lupin who's shocked and thrown off by Harry's arrival. He leaves the room to fetch Sirius. This gives Sirius time to process Harry's arrival and consider why Harry did not use the mirror instead. Surely, Sirius would have kept a close eye on the mirror should Harry have needed him, so its existence would be fresh in his memory. His first words to Harry should have been, "Are you alright, and is there something wrong with the mirror I gave you?"
It makes sense that Rowling wanted Harry to forget about the mirror because it would serve as a tragic learning opportunity for Harry, and would create an even sadder layer to Sirius' death. However, by having Sirius uncharacteristically also forget the mirror's existence, an unfortunate plot hole is created that is the root cause of the character's death. This really undermines the tragedy of the situation. That being said, the loss of Sirius Black is still one of the saddest moments in the entire series, and many fans will never really recover from it, much like Harry Potter himself.