Developer to self: "It'll take me about a week"

Developer to development lead: "It'll take about two weeks"

Development lead to project manager: "It'll take about 4 weeks"

Project manager to self: "It'll take double that plus 2 weeks"

Project manager to management: "It'll take 12 weeks"

Management to client: "It'll take 8 weeks"

Actual time taken: 16 weeks.