Back in December 2007, the main nuclear reactor that supplies the majority of the worlds supply of cobalt-60 and almost half of the world’s supply of Tech-99 had to be shut down.  This create a shortage of diagnostic and therapeutic nuclear medicine.  Apparently the Canadian government saw this possibility and started creation of two new reactors as backups; however, these had to be essentially scraped due to a miscalculation of the power coefficient of reactivity (PCR.)   A negative PCR means as the reaction increases, the design of the reactor is such that the reaction will slow down in a negative feedback loop.  The only problem was when they started tests, it actually had a positive reaction, meaning it would continue to build unless other safety measures were enabled (inserting rods for example to slow the reaction.)  Since the negative PCR was part of the safety spec, it was not allowed to continue. 

It’s an interesting read:  http://www.newscientist.com/article/mg20527431.400-nuclear-safety-when-positive-is-negative.html?DCMP=OTC-rss&nsref=online-news