It would have to be Gödel’s Incompleteness Theorem. Such a beautiful proof that shakes mathematics to its core.
The science communicator Veritasium made a nice video about it: https://youtu.be/HeQX2HjkcNo
I first learned about it in Douglas Hofstaedter’s masterpiece Gödel Escher Bach: An Eternal Golden Braid
Then change the title of the post to something open-ended like “How vulnerable is Lemmy to DDOS attacks?”. Taking out a major node which hosts many key communities is going to have an adverse impact.