cv::VideoWriter being bipolar
#include <opencv/highgui.h>
#include <iostream>

int main(int argc, char* argv[])
{
    if (argc < 3) { return -1; }

    cv::VideoCapture cap(argv[1]);

    std::string str = argv[2];

    int fourcc = CV_FOURCC('j', 'p', 'e', 'g');
    double fps = cap.get(CV_CAP_PROP_FPS);
    int width  = int(cap.get(CV_CAP_PROP_FRAME_WIDTH));
    int height = int(cap.get(CV_CAP_PROP_FRAME_HEIGHT));

    // COMMENT OUT THIS BLOCK TO MAKE THE PROGRAM CRASH
    // This code creates a variable on the stack and preforms
    // some operations with it.  This somehow alligns the memory
    // for the call "cv::VideoWriter vw(...)" in a way that
    // libavcoded expects and makes the call "vw << img" not crash.
    // Note that these variables are never used outside of this block.
    char * name_c = new char[100];
    strcpy(name_c, str.c_str());
    delete[] name_c;
    // END COMMENT OUT THIS BLOCK TO MAKE THE PROGRAM CRASH

    cv::VideoWriter vw(
        str,
        fourcc,
        fps,
        cv::Size(width, height));

    int fnum = 0;
    cv::Mat img;
    while(cap.grab() && cap.retrieve(img) && fnum < 30)
    {
        cv::imshow("img", img);
        vw << img;
        ++fnum;
    }

    return 0;
}

cv::VideoWriter vw(
        str,
        fourcc,
        fps,
        cv::Size(width, height));
00401290  fld         qword ptr [esp+50h]
00401294  add         esp,4
00401297  push        1
00401299  sub         esp,8
0040129C  mov         eax,esp
0040129E  mov         dword ptr [esp+60h],esp
004012A2  sub         esp,8
004012A5  fstp        qword ptr [esp]
004012A8  mov         dword ptr [eax],edi
004012AA  mov         dword ptr [eax+4],ebp
004012AD  push        6765706Ah
004012B2  lea         eax,[esp+0A8h]
004012B9  push        eax
004012BA  lea         ecx,[esp+5Ch]
004012BE  call        cv::VideoWriter::VideoWriter (401454h)

...

vw << img;
00401353  mov         edx,dword ptr [esp+40h]
00401357  mov         eax,dword ptr [edx+0Ch]
0040135A  lea         ecx,[esp+20h]
0040135E  push        ecx
0040135F  lea         ecx,[esp+44h]
00401363  call        eax
00401365  lea         ecx,[esp+14h]

cv::VideoWriter vw(
        str,
        fourcc,
        fps,
        cv::Size(width, height));
00401253  fld         qword ptr [esp+44h]
00401257  push        1
00401259  sub         esp,8
0040125C  mov         ecx,esp
0040125E  mov         dword ptr [esp+58h],esp
00401262  sub         esp,8
00401265  fstp        qword ptr [esp]
00401268  mov         dword ptr [ecx+4],eax
0040126B  push        6765706Ah
00401270  lea         eax,[esp+0A0h]
00401277  mov         dword ptr [ecx],edi
00401279  push        eax
0040127A  lea         ecx,[esp+54h]
0040127E  call        cv::VideoWriter::VideoWriter (401454h)

...

vw << img;
00401313  mov         edx,dword ptr [esp+38h]
00401317  mov         eax,dword ptr [edx+0Ch]
0040131A  lea         ecx,[esp+18h]
0040131E  push        ecx
0040131F  lea         ecx,[esp+3Ch]
00401323  call        eax
00401325  lea         ecx,[esp+0Ch]

// Working (with extra block)                       Crashes (without extra block)
    00401290  fld         qword ptr [esp+50h]      00401253  fld         qword ptr [esp+44h]
*** 00401294  add         esp,4
    00401297  push        1                        00401257  push        1
    ...
    004012A5  fstp        qword ptr [esp]          00401265  fstp        qword ptr [esp]
*** 004012A8  mov         dword ptr [eax],edi      00401268  mov         dword ptr [ecx+4],eax
*** 004012AA  mov         dword ptr [eax+4],ebp    0040126B  push        6765706Ah
*** 004012AD  push        6765706Ah                00401270  lea         eax,[esp+0A0h]
    004012B2  lea         eax,[esp+0A8h]           00401277  mov         dword ptr [ecx],edi
    004012B9  push        eax                      00401279  push        eax

#include <opencv/highgui.h>

void save(const std::string & str, cv::VideoCapture & cap)
{
    int fourcc = int(cap.get(CV_CAP_PROP_FOURCC));
    double fps = cap.get(CV_CAP_PROP_FPS);
    int width  = int(cap.get(CV_CAP_PROP_FRAME_WIDTH));
    int height = int(cap.get(CV_CAP_PROP_FRAME_HEIGHT));

    cv::VideoWriter  vw(
        str,
        fourcc,
        fps,
        cv::Size(width, height));

    int fnum = 0;
    cv::Mat img;
    while(cap.grab() && cap.retrieve(img) && fnum < 30)
    {
        cv::imshow("img", img);
        cv::waitKey(1);
        vw << img;
        ++fnum;
    }
}

int main(int argc, char* argv[])
{
    if (argc < 3) { return -1; }

    cv::VideoCapture cap(argv[1]);

    std::string str = argv[2];
    save(str, cap);
/*
    int fourcc = int(cap.get(CV_CAP_PROP_FOURCC));
    double fps = cap.get(CV_CAP_PROP_FPS);
    int width  = int(cap.get(CV_CAP_PROP_FRAME_WIDTH));
    int height = int(cap.get(CV_CAP_PROP_FRAME_HEIGHT));

    cv::VideoWriter  vw(
        str,
        fourcc,
        fps,
        cv::Size(width, height));

    int fnum = 0;
    cv::Mat img;
    while(cap.grab() && cap.retrieve(img) && fnum < 30)
    {
        cv::imshow("img", img);
        cv::waitKey(1);
        vw << img;
        ++fnum;
    }
*/
    return 0;
}