The name Flandre is a modern French term for Flanders, a region which makes out the Northern part of Belgium. They speak a dialect of dutch; Flemish. In Dutch you say it like 'Vlaanderen'.
A silhouette of Flandre occupies the front of Embodiment of Scarlet Devil's jewel case.
Flandre is one of the few characters to not appear in the background during Hopeless Masquerade, which would relate to her lack of leaving the Scarlet Devil Mansion.
Assuming her birth year by calculating backwards in the Gensokyo Timeline, she's supposed to be born around 1508 A.D. But the "canon-ness" of this assumption is unknown.
The Lævateinn that Flandre is holding in her older Sprite is very different from the Lævateinn that she's holding in her new Sprite. (See on right).
The spell card reference ("And Then Will There Be None?") to an Agatha Christie novel is definite. It's more speculative but still worth noting that the science-fiction genre saw Philip K. Dick write a novel called "Counter-Clock World" in 1967 and Frederik Pohl write "The Gold at the Starbow's End" in 1972.
On one end of the Lævateinn that Flandre is seen holding in Shoot the Bullet is very different compared to the Lævateinn that Flandre is seen holding in Embodiment of Scarlet Devil.
She used the phrase "that means, you can't use a continue!" to Marisa Kirisame, which was breaking the fourth wall.
In the Second Touhou Saimoe Tournament, the 8th game of the first round was a battle between Flandre and Suika Ibuki. Although the battle ended with Flan winning, this led to ZUN posting a rough drawing of Suika that hadn't been open to the public with a message saying "don't get defeated by a little sister that hasn't even reached the age of 500!".
The "crystals" on her wings look somewhat similar to the crystals shown on Patchouli Knowledge's spell card, Fire Water Wood Metal Earth Sign "Philosopher's Stone".
Strangely, if one uses a bomb against her in Embodiment of Scarlet Devil's Extra, she's shown to have normal wings in bat form.