Voldemort hid one of his Horcruxes inside Hogwarts. It seemed to be relying on security through obscurity as far as protecting it. But as we know, it ended up not being so secure because it wasn't so obscure - Harry Potter knew about the Room of Hidden Things.
My question is, the way Rowling describes this hints at the fact that Riddle had an assumption that very few people would ever stumble upon that room. WHY???
First, let's show Harry's theory on what Voldemort was thinking, how and why:
Here's Harry understanding Voldemort's thinking in HP7 once he figured out where the Diadem was (bold emphasis mine):
Tom Riddle, who confided in no one and operated alone, might have been arrogant enough to assume that he, and only he, had penetrated the deepest mysteries of Hogwarts Castle. Of course, Dumbledore and Flitwick, those model pupils, had never set foot in that particular place, but he, Harry, had strayed off the beaten track in his time at school – here at least was a secret area he and Voldemort knew, that Dumbledore had never discovered –
...and later on, with Harry explains the location to Ron/Hermione:
“He hid it exactly where I had my old Potions book, where everyone’s been hiding stuff for centuries. He thought he was the only one to find it. Come on.”
“And he never realized anyone could get in?” said Ron, his voice echoing in the silence. “He thought he was the only one,” said Harry.
Yet, from available evidence, that seems to be a completely wrong assumption, even at Voldemort's time.
Let's examine that last quote again, with emphasis on other info available:
“He hid it exactly where I had my old Potions book, where everyone’s been hiding stuff for centuries.” ...
“And he never realized anyone could get in?” said Ron, his voice echoing in the silence. “He thought he was the only one,” said Harry. “Too bad for him I’ve had to hide stuff in my time... this way,” he added. “I think it’s down here... “
They sped off up adjacent aisles; Harry could hear the others’ footsteps echoing through the towering piles of junk, of bottles, hats, crates, chairs, books, weapons, broomsticks, bats...
OK, so if there's a truckload (or train-load, by the description) of stuff in that room, isn't the only reasonable conclusion: "the room is a fairly popular destination with plenty of people who've found their way there in the past, and therefore, likely, plenty who will in the future"?
Doesn't matter what your arrogance level is, the evidence seems to point to only one conclusion - it's quite fine as a place to temporarily bury your dirt so teachers won't tag you for your misdeeds, but this is NOT the place to hide something that you want permanently safe!
So why did Voldemort make such a moronic mistake?