The number of new dams in countries with little or no experience in the design, construction and operation of dams increases every year. Lack of experience may lead to repetition of errors and serious mistakes in construction. Engineers may assume that the technology used to built small-scale dams can be used for putting up large dams. Shoddy workmanship is fairly common, so is "cutting corners" to complete the scheme in the shortest period possible. There may be a lack of cooperation between the various organizations involved in putting up a dam. Besides, dams will increasingly be built in less and less suitable places, because appropriate sites run out. Dams also fail because of overtopping during floods. The pressure applied to fragile geographical structures by vast mass of water impounded by a big dam can give rise to earthquakes. Sabotage caused by rebels or enemy forces can cause the loss of hundred of thousands of lives.
If dams are not maintained, they can flood. In 1996, an emergency deployment of 600,000 Chinese plugged holes made by rats and snakes in neglected dams on the Yangzi river. Nonetheless, 1100 people drowned and 2 million were left homeless by floods.
The number of large dams worldwide has climbed from just over 5,000 in 1950 to 38,000 in 1999.