In 1948, the White House was almost abandoned. After it had been burned by the British in 1814, and endured the stress of the addition of modern amenities like plumbing and electricity, it was on the verge of being condemned. Staircases were sagging, a fresco in the East Room was held in by scaffolding, and it was actually dangerous to be inside. In fact, the 1948 social season had to be cancelled (gasp!).
President Harry S. Truman told Congress that while it might be "more economical" to tear it down, that would be destroying a very important national monument. So in 1950, it was completely and totally gutted.
Every column, every wall, every single piece of the inside was taken out and put into storage. Check out these amazing photos of the White House during the reconstruction, taken from the National Archives' Flickr account. [National Journal via Digg]
Above, the Oval Study. Below, the Blue Room. Part of the floor was removed to install steel shoring columns.
Images from National Archives Flickr account