Not really anything that created a monster such as Mothman but...
The possibility that the Tunguska blast (June 30 1908)was caused by Tesla misfiring his death ray.
The Columbia space shuttle was the last scheduled mission dedicated to science. It carried dozens more experiments than a routine flight to the space
station.
There is the infamous Stanford Prison Experiment (1971)
www.prisonexp.org
Mendel and his pea plants, the experiments all went fine. but they came to an unfortunate end...if his works wasn't burned we'd be a couple decades
advanced in fields such as biology and understanding how genes work.
The Good Friday Marsh Chapel Experiment