Pittsburgh typically has three urban renewal mistakes which are considered to be the biggest tragedies.
1. Demolition of the Lower Hill to make way for the Civic Arena. A dense rowhouse neighborhood - with population densities of over 100,000 PPSM in some areas, was obliterated in the 1950s. The project displaced 8,000 residents, including a large proportion of the black community of Pittsburgh, inadvertently setting of white flight panics elsewhere in the city which suddenly saw a huge influx of poor black residents with nowhere else to go.
The arena has been demolished since 2011. The city, in a dumb move, decided to grant exclusive development rights to the Penguins, who played in the arena (but didn't own it) since 1967. They have been slow-walking development. The area is empty today, but the first apartments are expected soon.
2. Destruction of Old Allegheny City. The formerly independent city of Allegheny (Pittsburgh's North Side) first formed in a 36-block area surrounded by greenspace, which was later converted into parkland. In the 1950s, it was decided that the area - at that point still architecturally intact, if a bit downscale - would be "modernized" to help compete with the suburbs. The bulk of the southern half was converted into a giant shopping mall which opened in 1965, and basically ceased to be a mall by the 1990s. In addition they built two office buildings, four apartment towers with 840 units, and around 50 townhouses. The existing street grid was torn up, with a four-lane, one way circle road encircling the old core, meaning everyone had to drive around the area in a counter-clockwise fashion in order to go directly north/south or east/west (which meant people avoided the area entirely. A few historic buildings survived - mainly churches, but also the old planetarium and post office (which are now merged into the Pittsburgh Children's Museum) and the old Carnegie Library/Music Hall (which now houses the New Hazlett Theater). But many more were lost, including the old Allegheny City market house, and the entire stock of rowhouses, many of which dated from the 1830s.
Apparently accurate pre-urban renewal sketch
Demolition done - construction in progress.
3. The near-obliteration of the historic core of East Liberty. In the early 20th century, it was the retail hub of the City of Pittsburgh, and the third-largest "downtown" area in the state. However, with the rise of suburban malls, it began declining during the 1950s. A plan was hatched to do a less extreme version of what was done to Allegheny Center. The core main thoroughfare (Penn Avenue) was converted into a pedestrian mall. A new four-lane, one-way ring road (Penn Circle) surrounded the business district, and around half of the old historic buildings were demolished (a million square feet of retail in all) to make way for either the road changes or surface parking lots to entice the suburbanites back. At the same time, several large public housing complexes were built right on the fringes of Penn Circle, in part to provide better housing options for those displaced by the destruction of the Lower Hill. The area became a blighted near-ghetto within 10 years of the redevelopment, and only saw serious revival in the last decade.
There are many of lesser losses of individual buildings and areas. These include the demolition of an entire sub-neighborhood of Downtown to build Gateway Center/Point State Park, the demolition of the East Street Valley and much of Deutschtown to build I-579, and the destruction of much of Manchester, including its entire business district (though many beautiful Victorian houses remain). But the damage to these and other areas doesn't compare to the three outlined above.