Application Verifier is a feature of Windows for developers to use to check program correctness. Bruce says he turned on Full Page Heap, which makes every allocation go on a separate page, and tries to line it up so that the next page is not readable or writable. The idea is, if you go past the end of your memory allocation, the CPU raises a protection fault, and you catch the memory corruption bug at its source. Application Verifier can also do other things, like raise an error if your program ever passes an invalid HANDLE to a Windows system call.

Application Verifier is turned on by process name (i.e. "explorer.exe"). I think most of the time, developers turn on an Application Verifier feature, start just one process by that name, and then verify. But Bruce turned it on for a short-lived process that runs in a large build. So it runs thousands of times, and he hit this problem. It sounds like a great thing to fix, but I don't think it affects most developers using Application Verifier, and it definitely doesn't affect 'normal people' who are just using Windows on their laptop or desktop.