I just thought of something that I had never considered before in Harry Potter (actually it was my wife's idea, so if you find this interesting I give her full credit).
Was Scabbers in Ron's pocket during the Christmas feast in the 3rd book? If so, then 13 were dining together when Trelawney entered the room and Dumbledore rose to greet her, and thus she foreshadowed his death three books ahead of time.
This is the case, because Trelawney of course says (I'll paraphrase, because I don't have the book on me) "When thirteen dine, the first to rise is the first to die". Trelawney said this, because she thought that only twelve were dining at her arrival and didn't want to make the total thirteen. However, if Pettigrew were there then thirteen would have already been at the dining table when Dumbledore rose to greet her.
I think this is plausible and interesting because actually, quite a few things that Trelawney says comes true in the books even if it's not how the reader would expect. Except for her ridiculous things she predicts about Harry's death every class, she says almost entirely true predictions.
For example, she predicts correctly twice with her cards in the sixth book, once with Harry hiding behind a statue of armor, once with the lightning struck tower. Not to mention the several at the beginning of the third book that spooks the students.
Rowling also uses "fake" divination to foreshadow events. For instance, Harry and Ron's dream diary starts off foreshadowing the three Triwizard tasks. I thought this might have been another one of those times that I didn't pick up on before.
Since Ron carries Scabbers around most of the time, I figured it was likely. Is there any evidence in the book(s) about this?