In a multithreaded computing environment, hazard pointers are one approach to solving the problems posed by dynamic memory management of the nodes in a lock-free data structure.
Unfortunately, in the middle of this operation, another thread may have done two pops and then pushed A back on top, resulting in the stack (A → C).
The compare-and-swap succeeds in swapping `head` with `B`, and the result is that the stack now contains garbage (a pointer to the freed element "B").
Furthermore, any lock-free algorithm containing code of the form suffers from another major problem, in the absence of automatic garbage collection.
In between those two lines, it is possible that another thread may pop the node pointed to by this->head and deallocate it, meaning that the memory access through currentNode on the second line reads deallocated memory (which may in fact already be in use by some other thread for a completely different purpose).