Although it is true that testing should be rigorous thorough and systematic, this is not all there is to testing. There is a definite role for non-systematic techniques i.e. test based on a person's knowledge, experience, imagination and intuition. The reason is that some defects are hard to find using more systematic approaches, so a good ‘bug hunter’ can be very creative at finding those elusive defects.
In this section, look for the definitions of the glossary terms: error guessing, exploratory testing, and fault attack