I'm sure we've all been a part of projects were some phase - whether the beginning, middle, or end - was not properly planned out. It happens. But when that project is above all of our heads and rains chunks of metal over Australia during its reentry, it's a big deal.
NASA spent so much energy in the 60s and 70s designing, building, and launching SkyLab that they forgot to think about it's inevitable decline and descent back to Earth. Oops.
If you want to get really geeky, check out the full project history at the official NASA Skylab site. Beach reading!