ideas and make inferences. This includes the ability to detect and eliminate errors from your thinking, to relate product observations to quality criteria, and to build a compelling case for a particular belief or suggested course of action.
4) Practical thinking: The ability to put ideas into practice. This ability includes such skills as applying test tools and making test techniques and effort fit within the scope of the project.
Overall, thinking like a tester leads you to believe that things may not be as they seem. However things are, they could be different. We find that when the test process fails in the most damaging ways, the root cause is most likely to be tunnel vision. In other words, it's not that we ran 10,000 tests and should have run 10,001. It's that we failed to imagine an entire category of test; testing we wouldn't have performed even if we had twice the time and resources.
Ref: Notes from Lessons Learned in Software Testing: By Cem Kaner