As testers we are inclined to try to test everything, however factors such as timescales, available resources, technical complexities and costs can prevent this from happening.
The Risk profiles of a project or programme of work constantly change over the system development life-cycle. We all know that occasionally a swerve ball is thrown and we must react and adapt accordingly. Usually the project informs testing of the change and in return testing informs the project of the associated risk and details the probability of the risk involved. Testing then needs to be flexible and responsive to these changes to adapt, and to find a suitable solution to offer the best probability of success without compromising quality.