The mirror cracked from side to side; "The curse is come upon me," cried (lines 115-116)
Wham – there it goes. She slips up once, and looks outside, and the whole thing falls apart. She's cursed and doomed to die. Seems like the least fair punishment ever, especially with guys as hot as Lancelot wandering around. Still, even if it seems mysterious and bizarre, it's an intensely dramatic moment, really active and exciting.