It wasn't an unknown bug. This was a fix for a known bug. The point is that there was no test in place for memory isolation between users or processes. FYI,

It wasn't an unknown bug. This was a fix for a known bug. The point is that there was no test in place for memory isolation between users or processes. FYI, security testers call this type of testing negative testing, which is different from functional testing that is done to test an app works "properly". However, for an OS this test is not negative testing but functional testing if the OS is designed to enforce user and process isolation.