Defect cascading is a defect which is caused by another defect. One defect triggers the other defect. When a defect is present in any stage but is not identified, hide to other phases without getting noticed. This will result in an increase in the number of defects.
Let us understand this by an example.
You are designing the Login Module of a WebPage:
In phase 1 – You are designing Register User Module for Login and mobile number is mandatory but you can leave it blank due to a bug that gets unnoticed.
In Phase 2 – You will design the login form having username and password. The password is OTP which will be sent to User’s registered mobile number.
Now as Register module has a bug that mobile number can be left blank so this may lead to Login failure or maybe some system error or crash if a null mobile number is not handled. This is known as defect cascading.