This chapter examines three recent case studies drawn from widely differing domains. The first near-accident slipped through the minor cracks in the system; the second disaster fell through some yawning gaps in the defences; the third, unusually, had its origins in a single longstanding latent condition. The purpose of presenting these three case studies was twofold. First, to demonstrate in detail the variety of ways that a system's defences can be degraded and defeated to cause organizational accidents. The first and most obvious fact about all three of these accidents is that pre-existing and often longstanding latent conditions contributed to the breakdown of the system's defences. In all three events, the essential process of checking and reviewing the defences broke down. In the train derailment, the underlying problem was virtually undiscoverable before the event. It is thus a very special case. More usually, latent weaknesses in defences are potentially or even actually evident prior to a bad outcome.