She meant to invite just 15 people -- but by not checking her privacy settings, the event was made public -- and word got around quickly.
The girl's mother has canceled the party. But even if 1-percent of the people show up, it will be a nightmare for the small London suburb where she lives.
Police plan to beef-up patrols just in case a crowd of strangers overwhelms the area.
And in case you're wondering -- the girl's mother has taken away her daughter's computer.