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The following code causes two renderings of backgrounds and shifts one 4-5 pixels to the right if alpha blending.

#include <SDL/SDL.h>
#include <SDL_image/SDL_image.h>

int main(int argc, char*argv[])
{
     // Initialize SDL with the given driverName.
    SDL_Init(SDL_INIT_VIDEO);
     SDL_putenv("SDL_VIDEODRIVER=Quartz");

    // Create surfaces.
    SDL_Surface* pSurfaceOne = IMG_Load("/1.jpg");
     SDL_Surface *pSurfaceTwo = IMG_Load("/2.jpg");

    // Convert surfaces.
    SDL_Surface* pConversionSurface = SDL_CreateRGBSurface(SDL_SWSURFACE, 1, 
1, 32, 0xff000000, 0x00ff0000, 0x0000ff00, 0x000000ff);
    SDL_Surface* pSurfaceOneConverted = SDL_ConvertSurface(pSurfaceOne, 
pConversionSurface->format, SDL_SWSURFACE);
    SDL_Surface* pSurfaceTwoConverted = SDL_ConvertSurface(pSurfaceTwo, 
pConversionSurface->format, SDL_SWSURFACE);

    // Create window and display surface.
     SDL_Surface* pDisplaySurface = SDL_SetVideoMode(800, 600, 32, SDL_SWSURFACE);

    // Display
    int alpha = 0;
    while(true)
    {
        // Set alpha.
        alpha += 10;
        if(alpha > 255)
        {
            alpha = 0;
            SDL_Surface* temp = pSurfaceOneConverted;
            pSurfaceOneConverted = pSurfaceTwoConverted;
            pSurfaceTwoConverted = temp;
        }

        // Apply to surfaces and blit.
        pSurfaceOneConverted->format->Amask = 0;
        pSurfaceTwoConverted->format->Amask = 0;
        SDL_SetAlpha(pSurfaceTwoConverted, SDL_SRCALPHA, 255 - alpha);
        SDL_BlitSurface(pSurfaceTwoConverted, NULL, pDisplaySurface, NULL);
        SDL_SetAlpha(pSurfaceOneConverted, SDL_SRCALPHA, alpha);
        SDL_BlitSurface(pSurfaceOneConverted, NULL, pDisplaySurface, NULL);

        // Update the surface.
        SDL_Flip(pDisplaySurface);
    }
    return 0;
}

The assets are 800x600 jpegs.  The problem happens with the surface conversion code.