Q:

Debug Assertion Failed!  Expression: _BLOCK_TYPE_IS_VALID

I am getting this error message:

Debug Assertion Failed! 
Expression:_BLOCK_TYPE_US_VALID(pHead->nBlockUse)

while trying to do the following
#include <vector>
#include <algorithm>
using namespace std;

class NN
{
public:
    NN(const int numLayers,const int *lSz,const int AFT,const int OAF,const double initWtMag,const int UEW,const double *extInitWt);
    double sse;
    bool operator < (const NN &net) const {return sse < net.sse;}
};

class Pop
{
    int popSize;
    double a;
public:

    Pop(const int numLayers,const int *lSz,const int AFT,const int OAF,const double initWtMag,const int numNets,const double alpha);
    ~Pop();
    vector<NN> nets;
    void GA(...);
};

Pop::Pop(const int numLayers,const int *lSz,const int AFT,const int OAF,
         const double initWtMag,const int numNets,const double alpha)
{
    popSize=numNets;
    a=alpha;
    nets.reserve(popSize);
    for(int i=0;i<popSize;i++)
    {
        NN *net = new NN (numLayers,lSz,AFT,OAF,initWtMag,0,0);
        nets.push_back(*net);
    }
}

void Pop::GA()
{
...
        sort(nets.begin(),nets.end());
...
}

The error appears to be related to the sort function.  I check all instances of nets vector and they seem to be OK, having different sse's.  The funny thing is that I created a simpler case of the above code (see below) and it worked without any errors.  I am wrecking my brain.  Please help. 
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;

class Student
{
public:
    string name;
    double grade;
    Student(string,double);
    bool operator < (const Student &st) const {return grade < st.grade;}
};

Student::Student(string stName,double stGrade)
{
    name = stName;
    grade = stGrade;
}

int main()
{
    vector<Student> group;
    Student *st;
    st = new Student("Bill",3.5);
    group.push_back(*st);
    st = new Student("John",3.9);
    group.push_back(*st);
    st = new Student("Dave",3.1);
    group.push_back(*st);
    sort(group.begin(),group.end());
    for each(Student st in group)
        cout << st.name << " " << st.grade << endl;
    cin.get();
    return(0);
}

A:

The _BLOCK_TYPE_IS_VALID assertion gets fired, when you overwrite the header of an block allocated by new.  This happens when you slice objects, use dead objects, etc. 
You should have a look at your complete code, and try to work from the data you have in your debugger.  This short code snippet contains several 'curious' usage of C++, but no obvious point at which this produces the described error (at least for me). 

A:

from my experience- This type of error could be caused by Heap corruption.  so.. you must first check for memory leaks.  If you are using Visual studio use _CrtCheckMemory().