It's not always the "big bang" scenario that causes the problem

Business Continuity Management: An Overview

Plan testing, review and maintenance

Testing

A Business Continuity Plan is unreliable until it is exercised and has been proven workable, especially since false confidence may be placed in its integrity. The objectives of plan testing should be to

1) verify that it is practically workable and
2) familiarise staff with the operation of the plan.

This should be carried out at least once every 12 months, but for some businesses or processes a more frequent schedule may be necessary.

Review

The Business Continuity Plan must continue to reflect changes in the organisation and remain valid. A review must be carried out at least once every twelve months, or after significant organisational or structural changes, to ensure that the plan continues to reflect business strategies and needs.

Maintenance

A process should be established whereby the person responsible for Business Continuity Management within the organisation is informed of any changes and can incorporate them into the relevant business continuity plan(s).