I'll try to be the least biased as possible on the following post/essay/whatever.
First off, I'll focus evidently in my country (Chile) and the consecuences that the liberalization of land markets and lack of proper urban planning in Chile during the late '70s and '80s. This happened after the military government took over and adopted the neo-liberal model, and following the advice of "expert" economists (like A. Harberger), liberalized the boundairies of urban growth, which evidently caused an explosive growth of the cities, and specifically Santiago, Chile's Capital, that now has around 40% of the population in no more than 2% of the land. This fact plus the lack of investment in the subway system of Santiago and the lack of investment in regional and national commuter rail caused a growing urban chaos.
The basin in which Santiago was founded, presents one of the most fertile lands in Chile, many of these lands that were formerly agricultural are now the prey for land speculators, and with this, the downfall of the rural lifestyle (typical and modern). Many of these areas where filled with affordable housing far away from the city downtown, and be it for lack of information, or downright stupidity, many of these affordable housing neighborhoods ended in regular flood areas, causing a frequent social harm. Since these areas were developed without much planning and little infrastructure investment to allow good rainfall drainage, like in the case of the City center in which the rainfall collector drains date back to the 1920's. So now the latest governments have been forced to invest heavily in infrastructure, one of the areas that where more left aside during the Pinochet regime.
Since the subway system was practically left without any investment for growth, the majority of the public transport for Santiago came in the form of buses, that are highly contaminating and offer a very poor level of service. The growth of the city, created a need for a lot of buses and cars, besides the fact that the majority of the factories are located in Santiago, this caused the appearace of a smog layer that sometimes gets dangerously toxic due to lack of ventilation of the Basin of Santiago.
So, how many of these things could have been prevented with the existance of proper urban planning? The misery of many poor families that have to suffer winter after winter each year the flooding of their houses due to lack of investment in rainfall drainage infrastructure and the lack of scrupules of the building contractors that build in regular flood areas and the floodplains of many creeks and rivers; the destruction of fertile agricultural lands and the decay of the rural lifestyle; the air pollution, while not necesarily caused by the lack of urban planning, it got worse, just like one of the biggest problems in Chile; Centralism.
Comments are welcome, and of course further discusion of other cases.