What are the most common mistakes testers make?
Some of the most common mistakes I've seen testers make have to do with a basic understanding of the testing task and its place in the development lifecycle. Often new testers grossly underestimate both the complexity and skills required to adequately perform system testing. New testers may take a simplistic "push button" approach to testing vs. a systematic analytical approach. This is primarily due to a failure to fully grasp the importance of their role.
The tester's role is to apply market expectations upon a product and assess the system's ability to meet those expectations. The effectiveness of the testing function has major implications for the market success of the product. The accuracy with which testers predict and emulate end-user behavior often provides critical development feedback and early warning indicators. Conversely, if testers fail to adequately model the market, a heightened risk and vulnerability is created. A tester's failure to understand the significance of his role is perhaps the one mistake from which many others flow (i.e., carelessness, rework, etc.).
Testers are in a position to positively influence the success of the product and new testers need to develop a greater awareness of that fact.