Error guessing is the preferred method used when all other methods fail. Sometimes it is used to test some special cases. According to this method, errors or bugs, which do not fit in any of the earlier defined situations can be guessed. So test cases are generated for these special cases.

It is a very practical case wherein the tester uses his/her intuition and makes a guess about where the bug can be. The tester does not have to use any particular testing technique. However, this capability comes with years of experience in a particular field of testing. This is the reason that experienced managers can easily smell out errors as compared to a novice tester.

The history of bugs can help in identifying some special cases in the project. There is a high probability that errors made in a previous project is repeated again. In these situations, error guessing is an ad hoc approach, based on intuition, experience, knowledge of project, and bug history. Any of these can help to expose the errors.The basic idea is to make a list of possible errors in error-prone situations and then develop the test cases. Thus, there is no general procedure for this technique, as it is largely an intuitive and ad hoc process.

For example, consider the system for calculating the roots of a quadratic equation. Some special cases in this system are as follows:

• What will happen when $a=0$ ? Though we do consider this case, there, there are chances that we overlook it while testing, as it has two cases:

  (i) If $a=0$ then the equation is no longer quadratic.

  (ii) For calculation of roots, division is by zero.

• What will happen when all the inputs are negative?

• What will happen when the input list is empty?