An unlikely but very bad event (technical word)

One phrase that's gaining widespread adoption is black swan or black swan event. This refers to unlikely but potentially catastrophic events, especially ones where the risk is under appreciated. From Wikipedia:

The black swan theory or theory of black swan events is a metaphor that describes an event that comes as a surprise, has a major effect, and is often inappropriately rationalized after the fact with the benefit of hindsight.

… the "black swan theory" refers only to unexpected events of large magnitude and consequence and their dominant role in history. Such events, considered extreme outliers, collectively play vastly larger roles than regular occurrences
…
Black swan events were introduced by Nassim Nicholas Taleb in his 2001 book Fooled By Randomness, which concerned financial events. His 2007 book The Black Swan extended the metaphor to events outside of financial markets. Taleb regards almost all major scientific discoveries, historical events, and artistic accomplishments as "black swans"—undirected and unpredicted. He gives the rise of the Internet, the personal computer, World War I, dissolution of the Soviet Union, and the September 2001 attacks as examples of black swan events.

In the comments above, @RegDwighт suggests the acronyms MCA and DBA, which stand for the synonymous maximum credible accident and design basis accident, refer to unlikely events that are expected to occur frequently enough that engineers must design around them.

Recent events clearly illustrate the difference between a black swan and an MCA.

The tsunami far exceeded the MCA determined by the nuclear regulator, setting the stage for the black swan meltdown at Fukushima.

EDIT: Opened answer up to community wiki, since I don't want to have to accept my own answer.

I would call it a fluke, although after checking some dictionaries I see they associate it only with good events (I don't).

Also a freak occurrence.