Microsoft's Allocator (Score 1)

The reason why it is mostly a non-issue with Microsoft is because their RTL's alloc implementation is protected against double frees. This makes it a tiny bit slower.

But microsoft tried to remove this protection in one of their Visual Studio services packs - the result - Microsoft's ( and other's ) programs crashing randomly all over the place. They quickly reversed the 'optimization'

More worryingly -that means that alot of programs are actually relient on that thin safety net!