On numerous ocassions have I found people making mistakes in usage of the memory allocation routines acceptable to C++. Primarily between new, delete and malloc, free (and not to mention the array forms of new and delete).
A mistake can be very dangerous from a memory leak to undefined behavior that may make your computer dissappear! :D Of course, I am kidding but it is still dangerous.
Take a look at this FAQ entry that I posted on Codeguru regarding the topic - C++ Memory Management : What is the difference between malloc/free and new/delete?
Hope you find it useful.