An unforeseen event causing great loss, upset or unpleasantness of whatever kind.
An unexpected natural or man-made catastrophe of substantial extent causing significant physical damage or destruction, loss of life or sometimes permanent change to the natural environment.
A thing or occurrence which initially causes fear, distress, etc. but which is subsequently recognized as being no cause for concern.
A warning signal (such as the call of sentry, the ringing of a bell, or the shriek of a siren) which turns out to have been given erroneously.