Are you using smart pointers such as Boost shared_ptr? If so, check if you are directly using the raw pointer anywhere by calling get(). I've found this to be quite a common problem.
For example, imagine a scenario where a raw pointer is passed (maybe as a callback handler, say) to your code. You might decide to assign this to a smart pointer in order to cope with reference counting etc. Big mistake: your code doesn't own this pointer unless you take a deep copy. When your code is done with the smart pointer it will destroy it and attempt to destroy the memory it points to since it thinks that no-one else needs it, but the calling code will then try to delete it and you'll get a double free problem.
Of course, that might not be your problem here. At it's simplest here's an example which shows how it can happen. The first delete is fine but the compiler senses that it's already deleted that memory and causes a problem. That's why assigning 0 to a pointer immediately after deletion is a good idea.
int main(int argc, char* argv[])
{
char* ptr = new char[20];
delete[] ptr;
ptr = 0; // Comment me out and watch me crash and burn.
delete[] ptr;
}
Edit: changed delete to delete[], as ptr is an array of char.