What realisation in Harry Potter blew your mind the most?
Two realisations in the first book, Harry Potter and the Sorcerer’s Stone, really blew my mind.
In the first book, Professor Snape meets Harry in his class for the first time. This scene plays out:
“Potter!” said Snape suddenly. “What would I get if I added powdered root of asphodel to an infusion of wormwood?” Powdered root of what to an infusion of what? Harry glanced at Ron, who looked as stumped as he was; Hermione’s hand had shot into the air. “I don’t know, sir,” said Harry. Snape’s lips curled into a sneer. “Tut, tut — fame clearly isn’t everything.”
-Harry Potter and the Sorcerer’s Stone
Some time ago I was quite shocked to find out there was a deeper meaning in the words, more specifically, the names of the ingredients Snape was using. 
One of the many things J.K Rowling did well in her HP books was the use of subtle foreshadowing.
At that point in time when Snape first picked on Harry in class, Harry did not know anything about Snape and his mother; he did not know that he loved her very much, that he was completely heartbroken when she died and that he begged Dumbledore to protect Lily - and all these things readers learned at the end of the last book.
Harry thought Snape was just picking on him, but as I said there was a deeper meaning to his words.
What would I get if I added powdered root of asphodel to an infusion of wormwood?”
“Asphodel” is a flower, and not just any old flower, it is a member of the Lily family.
Many asphodels are popular garden plants, which grow in well-drained soils with abundant natural light. Now placed in the family Asphodelaceae, the genus was formerly included in the lily family (Liliaceae). 
Harry’s mother’s name was Lily. Coincidence? No.
Asphodel also means my regrets follow you to the grave. 
Now to “wormwood”.
Wormwood means something that is bitter, and it is often associated with regret or bitterness.
something bitter or grievous 
So, what can be made of that? He was just saying “bitter Lily”?
It is believed that that was Snape’s way of telling Harry that he was bitterly sorrowful and regretful of his mother's (Lily’s) untimely death.
The second realisation has to do with Fred and George Weasley.
…….the Weasley twins were punished for bewitching several snowballs so that they followed Quirrell around, bouncing off the back of his turban.
-Harry Potter and the Sorcerer’s Stone.
The twins made snowballs bounce off the back of Quirrell's turban, the back of his head.
The twins had repeatedly hit Voldemort in the face.
Think about that for a minute.