Ninety-Ninety Rule

Ninety-Ninety Rule n.  "The first 90% of the code accounts
   for the first 90% of the development time.  The remaining 10% of
   the code accounts for the other 90% of the development time."
   Attributed to Tom Cargill of Bell Labs, and popularized by Jon
   Bentley's September 1985 "Bumper-Sticker Computer Science"
   column in "Communications of the ACM".  It was there called
   the "Rule of Credibility", a name which seems not to have stuck.