This publication explores the implications of decommissioning in the light of unexpected events and the trade-off between activities to reduce them and factors militating against any such extra work. It classifies and sets out some instances where unexpected findings in a decommissioning programme led to a need to either stop, or reconsider the work, re-think the options, or move forward on a different path. It provides practical guidance in planning and management of decommissioning taking into account unexpected events. This guidance includes an evaluation of the experience and lessons learned in tackling decommissioning that is often neglected. Thus it will enable future decommissioning teams to adopt the relevant lessons to reduce additional costs, time delays and radiation exposures.