All the Pottermore articles on Hogwarts are collected in a book called Hogwarts: An Incomplete and Unreliable Guide.
I understand it's a catchy title, but is there actually any information in there which is "unreliable" or conflicts with known canon?
I'm assuming that this is riffing off the section in Harry Potter and the Goblet of Fire where Hermione discuses the limitations of the text of Hogwarts : A History.
“It’s all in Hogwarts, A History. Though, of course, that book’s not entirely reliable. A Revised History of Hogwarts would be a more accurate title. Or A Highly Biased and Selective History of Hogwarts, Which Glosses Over the Nastier Aspects of the School.”
“What are you on about?” said Ron, though Harry thought he knew what was coming.
“House-elves!” said Hermione, her eyes flashing. “Not once, in over a thousand pages, does Hogwarts, A History mention that we are all colluding in the oppression of a hundred slaves!”
The image you've included in the question shows the title of the book would be Hogwarts : A complete and Reliable Guide without the post-publication additions by an unknown editor.
It is an indication that the reader shouldn't take the information inside to be wholly reliable or complete.
Two examples come to mind:
1) Slytherin is described as being forced out of the school, but the Sorting Hat, who was present, describes Slytherin as having departed (seemingly of his own accord).
Sorting Hat:
These differences caused little strife
When first they came to light,
For each of the four founders had
A House in which they might
Take only those they wanted, so,
For instance, Slytherin
Took only pure-blood wizards
Of great cunning, just like him,
And only those of sharpest mind
Were taught by Ravenclaw
While the bravest and the boldest
Went to daring Gryffindor.
Good Hufflepuff, she took the rest,
And taught them all she knew,
Thus the Houses and their founders
Retained friendships firm and true.
So Hogwarts worked in harmony
For several happy years,
But then discord crept among us
Feeding on our faults and fears.
The Houses that, like pillars four,
Had once held up our school,
Now turned upon each other and,
Divided, sought to rule.
And for a while it seemed the school
Must meet an early end,
What with dueling and with fighting
And the clash of friend on friend
And at last there came a morning
When old Slytherin departed
Hogwarts Guide:
What is certain is that by the time Slytherin was forced out of the school by the other three founders, he had decided that henceforth, the Chamber he had built would be the lair of a monster that he alone – or his descendants – would be able to control.
2)Hermione is described as taking four minutes to be sorted, but in the book, it implies she was sorted rather quickly (with only Seamus being pointed out as having taken a long time.)
Hogwarts Guide:
An archaic Hogwarts term for any new student whose Sorting takes longer than five minutes. This is an exceptionally long time for the Sorting Hat to deliberate, and occurs rarely, perhaps once every fifty years. Of Harry Potter’s contemporaries, Hermione Granger and Neville Longbottom came closest to being Hatstalls. The Sorting Hat spent nearly four minutes trying to decide whether it should place Hermione in Ravenclaw or Gryffindor.
Sorcerer's Stone:
Sometimes, Harry noticed, the hat shouted out the House at once, but at others it took a little while to decide. “Finnigan, Seamus,” the sandy-haired boy next to Harry in the line, sat on the stool for almost a whole minute before the hat declared him a Gryffindor. “Granger, Hermione!” Hermione almost ran to the stool and jammed the hat eagerly on her head. “GRYFFINDOR!”
Which implies it took Hermione quicker than Seamus Finnigan.
