Something that happened in the past. No one talks about it, no one knows exactly what it was. But everyone agrees that it was something embarassing or bad.
From "Calvin and Hobbes".
(At a 20-year High School Reunion)
Wife: Why is everyone looking at you funny?
Husband: They must not have forgotten the noodle incident.
Wife: You never told me about that. What was it?
Husband: ... ... it was... the noodle incident. (looks away shamefully)
Wife: (goes off to find someone to explain this to her)