I have just read an article that claims the demise of the Boeing Starliner’s first flight failed due to a lack of testing; specifically end-to-end testing. (https://www.engadget.com/2020/02/29/boeing-starliner-failed-first-flight-report/)
You can be sure that extensive tests were performed on all the various components and phases of the mission. However, no end-to-end test was performed to ensure each phase correctly ‘connected’ to the next.
The importance of end-to-end testing equally applies to business-critical applications. Many of our customers will often talk about ‘Flow’ testing. A Flow is an end-to-end business process, such as Order To Cash. This Flow, or process, involves many steps from taking an order, through manufacturing, distribution, invoicing and receiving payment. Each step in the process can be, and is, tested in isolation, but until the whole process from start to finish is verified, you cannot be sure that you can take an order and, ultimately, get paid for it.