It will most likely not happen? It's absolutely possible to write a program that forks and both forks overwrite 99% of shared memory pages. It almost never happens, which is GP's point, but it's possible and the reason it's a fragile hack.
What usually happens in practice is you're almost OOM, and one of the processes running in the system writes to a page shared with another process, forcing the system to start good ol' OOM killer.
What usually happens in practice is you're almost OOM, and one of the processes running in the system writes to a page shared with another process, forcing the system to start good ol' OOM killer.