Query regarding iterators and vector

I am new STL programming.
I have a query regarding vectors. If I am iterating over a vector
using a iterator, but do some operations that modify the size of the
vector. Will the iterator recognize this?

I wrote the following program to test this out.

#include <fstream>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

int main()
{
    vector<int> a;

    a.push_back(1);
    a.push_back(2);
    a.push_back(3);
    cout<<"Vector test begin"<<endl;
    vector<int>::iterator iter0;
    for(iter0=a.begin(); iter0!=a.end(); iter0++)
    {
        cout << "\n value: " << (*iter0)<<endl;
        if(*iter0 == 2)
        {
            a.push_back(4);
            a.push_back(5);
        }
    }
    cout<<"Vector test end";
}

I am getting the following output. Could somebody please explain what
is happening in my program. I am thoroughly confused.

---------------------------------------Output------------------
Vector test begin

 value: 1

 value: 2

 value: 3

 value: 4

 value: 0

 value: 41

 value: 1

 value: 2

 value: 3

 value: 4

 value: 5

 value: 4

 value: 5
Vector test end

----------------------------------------------End of
Output--------------------------------------