Label the likelihood and impact high, medium or low, with the definition of each level being specific to an organization and even a project. Volumes have been written on risk analysis, but this is where you must apply some judgment. For a project like the moon landing, NASA tried to address every foreseeable event. However, for most projects you will encounter in business, it is sufficient to address only those circumstances with a high likelihood and a high impact or a medium likelihood and a high impact. Your plan should include trigger events that will alert you that a particular problem may be occurring.
Some projects are inherently error-prone.
Any of these added risks should be a definite candidate for inclusion in the mitigation plan. Better yet, avoid or reduce the risks by plotting your course through good development practices. Limit the impact of new techniques or technologies to components that are not mission-critical, and get training and practice in advance. Use pilot projects. Avoid overloading on features.