Planning fallacy
The Sydney Opera House was expected to be completed in 1963. A scaled-down version opened in 1973, a decade later. The original cost was estimated at $7 million, but its delayed completion led to a cost of $102 million.
University of Sydney Library
The need for one location with enough space for the Sydney Conservatorium of Music prompted the State Government to step in and rebuild the Greenway building at a cost of A$144 million, and so; in 2001 upon completion of the works, the entire Conservatorium (including the library) moved back into what is now a remarkable renovation of a historical space.