According to legend, a rich widow named Berta di Bernardo left 60 coins in her will for the construction of a campanile, a bell tower, for St. Mary's Cathedral in Pisa. A 55-meter tower was planned, belvedere style so that people could walk up the outside and enjoy the view. Ground was broken August 9, 1173.
The architect/builder is unknown. Either he was simply undistinguished, or he was careful to keep his name out of the public eye once the project was begun in order to avoid the inevitable ridicule. Pisa was built at the junction of two rivers, the Arno and the (now dried and gone) Auser; when the builder dug a 3-meter deep hole for the foundation, he found only river clay and sand. This lack of bedrock did not deter him. He proceeded to build what was designed to be a 14,000-ton structure on top of this soft foundation; by the time he got to the third level in 1178, it had already started to list to one side.
They had no way to change the ground on which it stood, and they didn't want to give up the project, so they built the next floors with one wall higher than the other, so that the upper floors wouldn't slant.
A series of financial problems and wars with neighboring city-states (mostly Florence, but Pisa also lost a major conflict with Genoa) meant the campanile wasn't finished until 1319. Nothing they did could fix the increasing list. But then, nothing they did would have. Adding the bells? More leaning. Deciding to add an impressive 3.5 ton additional bell? More leaning. Even so, the tower was only leaning by five degrees in 1837, when Alessandro della Gherardesca decided to turn an embarrassment into a tourist attraction by exposing the famous flawed foundation. He dug around the base...below the water table. The space flooded, the clay and sand softened more, and over the course of a few days, the tower leaned another meter!
In 1934, the idea arose to drill holes in the foundation and force grout into them. It looked good on paper; in reality, it increased the tilt. Tinkering in 1966 and 1985 just made it worse, and the tower was closed to the public in 1990.
Years later, the combination of a steel corset around the base, concrete, and counterweights were supposed to be the ultimate fix. They weren't, although they did almost cause the whole tower to break apart from stress. Finally, someone decided to go to the root of the problem. They dug away at the foundation on the other side of the tower, hoping it would lean back the way it had come. A little over 800 years of tinkering since the problem first appeared, and not only was the tilt reduced, but the tower was stabilized and re-opened to the public. Once again, tourists can climb the tower and take photographs looking out over Pisa, rather than just ground-based pictures of them appearing to hold up the tower with their hands.