Normal Accidents

ValuJet 592 is the flight that went down in the Everglades in 1996, caused by a fire in the cargo compartment.

In William Langewiesche's article, The Lessons of ValuJet 592, he suggests three type of aircraft accidents. Most common, "procedural" caused by obvious mistakes such a taking off with ice on the wings. The second type is what he calls "engineered" such as the TWA Boeing that suffered an explosion in a near empty fuel tank. The third type, what Langewiesche calls a "system accident" what has also been called by Charles Perrow a "normal accident. The fire that brought down flight 592 was caused by oxygen tanks that ironically are intended to make flying safer. The chain of events that led to them being placed aboard the aircraft was the classic error chain.

There is an good explanation of a normal accident from NASA here (pdf) .
In a system accident the root cause is difficult to determine, there is more then one, the familiar the error chain/ Causes can be complex and interrelated. But...analysis of close calls and mishaps is an effective way to break error chains.

Small mishaps, and daily incidents are clues to possible problems. At the first drill a crewmember doesn't know where to muster. The new mate on watch can't operate the ARPA. A port state control inspector isn't satisfied with your passage plan. Each event should be analyzed, is your familiarization effective? Is you passage plan adequate? Your entire program should be under constant scrutiny for loopholes where an error can slip through.