In Chapter Six of Prisoner of Azkaban we have the following passage:
"Really, what has got into you all today?" said Professor McGonagall, turning back into herself with a faint pop, and staring around at them all. "Not that it matters, but that's the first time my transformation's not got applause from a class."
Everybody's heads turned toward Harry again, but nobody spoke. Then Hermione raised her hand.
"Please, Professor, we've just had our first Divination class, and we were reading the tea leaves, and –"
"Ah, of course," said Professor McGonagall, suddenly frowning.
"There is no need to say any more, Miss Granger. Tell me, which of you will be dying this year?"
Everyone stared at her.
"Me," said Harry, finally.
"I see," said Professor McGonagall, fixing Harry with her beady eyes. "Then you should know, Potter, that Sibyll Trelawney has predicted the death of one student a year since she arrived at this school. None of them has died yet. Seeing death omens is her favorite way of greeting a new class. If it were not for the fact that I never speak ill of my colleagues –"
As soon as Hermione mentions the Divination class McGonagall understands why everyone was too preoccupied to be impressed by her transformation. So if the same thing happens every year, why was this the first time that no one applauded the transformation? Surely this was not the first time ever that the first Divination class and the first Transfiguration class were back to back.
So why was it that specifically in Harry's year everyone was too overwhelmed to care about McGonagall's transformation, but in every other year the students (or at least one student) managed to focus on McGonagall's transformation enough to applaud it?