Despite the exhaustive efforts put into making critical systems "failsafe", fail they do. The best example of this that I can think of is the Three Mile Island accident. A modern reactor design with considerable "defence in depth" suffered an incredibly unlikely series of failures, omissions, and errors which conspired to create what could have been a Chonobyl. The ultimate scenario was avoided, not through good design and procedures, but people on the scene thinking things through.

Steam condensate water hammer blows out gasket in 8 inch pipe [US DOE]

General
Aviation
Aviation is an area where failures are generally well studied and copious amounts of information is available on the web. In particular, the manner in which highly redundant systems still fail is enlightening.
Reactor Accidents
Inside sarcophagus, Chornobyl NPP Unit 4 control room, as seen during tour. Photographer: Dennis Kreid 1996 May [US DOE]

Artur Korneev, Deputy Director of Shelter, viewing a portion of the Central Reactor Hall showing cylindrical pipe roof members, Mammoth Beams and portions of the destroyed refuelling machine, Chornobyl NPP. Photographer: Unknown. Spring 1997. [US DOE]

Artur Korneev, Deputy Director of Shelter Object, viewing the elephants foot lava flow, Chornobyl NPP. Photographer: Unknown. Fall 1996. [US DOE]

1957 Windscale Pile 1 Nuclear Reactor Accident (air cooled reactor)
This was the first significant release of radioactive material from a nuclear reactor. It occurred while the graphite moderator was being annealed to release Wigner energy (distortion of atomic structure that results from the continual bombardment of neutron radiation, a effect that was poorly understood at the time). The energy was released too quickly and the uranium metal fuel and graphite moderator started burning. The fire wasn't immediately noticed due to inadequacies in the design of the monitoring system.

While this accident occured over 40 years ago, cleanup still hasn't been completed - in part due to concerns that further uncontrolled releases of Wigner energy may occur and restart the core fire.

1979 Three Mile Island Unit 2 (TMI-2) Reactor Accident (pressurised water cooled reactor)
1986 Chornobyl Unit 4 Nuclear Reactor Explosion (RBMK reactor)
This is the stuff of nightmares - not the so much from the "oooh... dirty nuke go bang, bad bad" perspective, but the total devastation of the plant, the impact it's had on the Ukraine, and the sheer scale of the cleanup operation - an operation that can essentially never complete. One official web site mentions that around 25% of the Ukrainian state budget is spent on remediation of Chornobyl - and that ignores foreign aid.

Something like 8 tones of nuclear fuel was ejected from the reactor in the surrounding area, much of the heavy material falling close by, but a cloud of radioactive dust and gases rose around 10km into atmosphere, contaminating more than 155,000 sq km.

The three other reactors at the site have all been operated since the explosion in Unit 4. Unit 2 was closed after a fire in the turbine building in 1991, Unit 1 was closed in 1996. Unit 3, which is part of the same building as Unit 4, and shares some facilities, was still operating as of 2000.

An image that haunts is that of "lava flows" of melted nuclear fuel emerging from pipework, a ghostly worker looking on. In another image a rusty galvanised steel bucket that appears to contain a couple of twisty turds. They're fuel rods ejected by the exploding reactor, and collected by personnel repairing the ventilation stack (the big chimney).

Other Incidents
Damage resulting to Channel Tunnel wall after fire of 1996

Wagon damaged in Channel Tunnel fire of 1996

1947 S.S. Grandcamp Ammonium Nitrate Explosion - Texas City Habour:
A fire and explosion in a ship containing 2300 tones of fertiliser killed 576 people in Texas City - nearly 4% of it's population at that time. It's considered the worst industrial accident in American history.

As is common in incidents of this sort, there were a large number of contributing factors, any of which could have at least reduced the scale of the disaster, if not prevented it altogether.

1983 radioactive pellets from radiotherapy machine mixed with scrap steel in Juarez, Mexico.
A derelict radiotherapy machine is broken up for scrap. The Cobalt-60 pellets end up being smelted with other recycled steel and turned into 600 tones of reinforcing rod and table legs, which are then shipped to Arizona, California, Colorado, New Mexico, and Texas. The error was only discovered when a truck load of reinforcing bars sets off radiation detectors at the Los Alamos National Laboratories six weeks later.
1987 radioactive pellets from radiotherapy machine stolen by junk collectors in Goiana, Brazil.
In an almost identical incident, a gamma-ray radiotherapy unit in an abandoned building is stripped by junk collectors, and the source is cut open, extracting powdered Caesium-137. The powder is rubbed into the skin of several people, including children. The incident comes to light when they start showing up in local hospitals with symptoms of radiation poisoning. 4 people died, innumerable people were exposed, and around 3500 cubic meters of radioactively contaminated waste was generated.
1996 Channel Tunnel Fire

Andrew McNamara <andrewm@connect.com.au>