The StackGhost XOR Cookie methods of hindering exploits do not always detect the corruption of the stack. If the attacker's return pointer manages to align correctly after being XORed with the cookie, execution will be transfered in an unpredictable manner.
Execution may divert to a random but legal stretch of code and cause data corruption. Of course, a successful attack may have the same chance of causing data corruption since no cleanup code will be called anyway.