If not little known, at least little remembered. ;) I've collected here eleven (in my opinion) of the most interesting and useful ones. Plus my comments on some of them.
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There is no such thing as a pureblood family. Every wizarding family has a muggle or a few among them. Some just choose to cross them over in their family tree and pretend they don't exist and instead claim themselves as purebloods.
^I can see Blacks and Malfoys doing that. But makes me wonder why were the Crouches noted as purebloods then, as evidently the father and son were not passionate about purity of blood. The father fought Voldemort's fire with fire and the son in the trial disregarded whatever devotion he might have had for Voldemort and instead of proudly suffering, was dying in Azkaban within a year even. His reasons to serve Voldemort seemed to be whatever else but not in the purifying the wizarding "race". And how about the Longbottoms who also fought Voldemort?
Perhaps they just didn't bother commenting on the general impression of their blood purity because it really didn't matter to them and speaking of it wouldn't have made any difference to the battle? After all the most we heard of them in the blood-purity terms was during the First War.
Wizards and witches have a much longer life expectancy than muggles. Dumbledore is 150 years old and McGonagall is sprightly 70-years old.
^I love this to bits! Gives such a deliciously long timeline and lots of possibilities for my original character and his family. <3
Hogwarts does not accept foreigners but serves only Britain and Ireland.
There's a magic potion for developing photographs and it's that potion that makes the photos also move.
^This is what I'd wondered for a long time. I was glad to find out.
A Squib child can be born to any witch or a wizard regardless of how pure the blood. The other of the parents can be even a muggle. Still Squibs are rare because magic is a dominant and resilient gene.
Most magical children are being home schooled but some may go to a muggles' primary school before Hogwarts. Purity of blood is irrelevant in this matter.
with references to Rowling's official words
Fred and George did not pay attention to Peter Pettigrew on the Marauders' Map because they hadn't heard the story of Peter and Sirius and even if they had, they'd have no reason to think that one is the man murdered years ago. Plus, they used the Map for their own mischief and thus focused on only the places they needed to. And finally, you must remember there were hundreds of little dots moving on the Map at any given time and the twins did not know all the students by the name.
^My personal off-topic conclusion: This answer also reveals her "there are a thousand students at Hogwarts" indeed is just her being bad at thinking in numbers like she's admited to. As in hundreds of dots on that Map means just hundreds of students attending Hogwarts, and not a thousand. Since both numbers are her words outside the books which do not confirm the matter in any way, either is equally official, and all you need to decide is which to you sounds more realistic amount of students for such a horribly limited number of teachers, especially when Hogwarts is claimed to be "the finest school for witches and wizards".
And even with magic, which amount is more comfortable and sensible to devide into only four Houses seeing to that each House's members are requied to stay after hours in and sleep in the same House area...and eat on the same tables while the lunch break can't last too long...
My vote goes for the hundreds being the more truthful amount of students in total at Hogwarts.)
Was Snape hiding under the Invisibility Cloak on the night the Potters died? No, he wasn't.
^I'd never even heard of this rumour befora, but thought it interesting.
The Lestranges did not know about the Prophecy.
^Whether or not Crouch Jr. did remains unknown.
It's very rare (but possible) that magic shows in a person late in life instead of in their childhood. Almost always it shows in a person before age 11.
No one knows where magic comes from.