Perhaps all PMs know already that: a call or a long mail received, informing the PM that one of the project team members caused a critical issue on Production system that has very high impact.
During the execution phase of the project the task criticality is very high and potential impact of an human error is huge - starting with financial lost, business impact and ending with even worse consequence: lost of customer trust.
For the project tasks defined during planning phase PM should have an quality control mechanism in place. But during the project execution phase there are always appearing additional tasks that were not originally in project scope / WBS. Fore these few rules / hints: 1. PM should have full control over the customer requirements. Never customer should give tasks directly to project team members. 2. Each new task should be analyzed from quality perspective and risk/impact analysis of each new tasks/customer request is mandatory. 3. Avoid tracking the new tasks only via email communication - Use task list tracker (simple excel) 4. Analyze the suitability of project resources assigned to new tasks - remember that these were not planned and can be much more complex - architect review will sometimes be needed 5. Execution plan of each new task should be reviewed and approved by customer. 6. Keep in mind financial impact on project budget.