If you are involved in software development then recalling a basic testing principles once again is not a waste of time. So here are the principles:

  1. A necessary part of a test case is a definition of the expected output or result.
  2. A programmer should avoid attempting to test his or her own program.
  3. A programming organization should not test its own programs.
  4. Any testing process should include a thorough inspection of the results of each test.
  5. Test cases must be written for input conditions that are invalid and unexpected, as well as for those that are valid and expected.
  6. Examining a program to see if it does not do what it is supposed to do is only half the battle; the other half is seeing whether the program does what it is not supposed to do.
  7. Avoid throwaway test cases unless the program is truly a throwaway program.
  8. Do not plan a testing effort under the tacit assumption that no errors will be found.
  9. The probability of the existence of more errors in a section of a program is proportional to the number of errors already found in that section.
  10. Testing is an extremely creative and intellectually challenging task.

I recommend reading a book «The Art of Software Testing» by Glenford j. Myers, Tom Badgett and Corey Sandler (ISBN: 978-1-118-03196-4).