What does the term Pesticide paradox' refer to?

The term ‘Pesticide paradox’ refers to the phenomenon where the effectiveness of test cases that are repeated and focused on the same scenarios decreases over time because they tend to find the same defects or no defects at all. This is because the system under test becomes more resistant or immune to the existing test cases, just like pests become more resistant or immune to pesticides over time. To overcome the pesticide paradox, test cases should be regularly reviewed and updated to cover new or changed requirements, scenarios, risks, or defects. Test cases should also be designed to cover different aspects and perspectives of the system under test, such as functionality, usability, performance, security, etc. You can find more information about the pesticide paradox in A Study Guide to the ISTQB® Foundation Level 2018 Syllabus, Chapter 4, Section 4.11.