When creating two surfaces and blitting them onto the other, SDL's internal reference counting fails, and one of the surfaces is not freed when calling SDL_FreeSurface.

Example code :

SDL_Surface *s1 = SDL_CreateRGBSurfaceWithFormat(0, 640, 480, 32, SDL_PIXELFORMAT_ARGB8888);
SDL_Surface *s2 = SDL_CreateRGBSurfaceWithFormat(0, 640, 480, 32, SDL_PIXELFORMAT_ARGB8888);

SDL_BlitSurface(s1, NULL, s2, NULL);
SDL_BlitSurface(s2, NULL, s1, NULL);

SDL_FreeSurface(s2);
SDL_FreeSurface(s1);

With this example, s1 is not freed after calling SDL_FreeSurface, its refcount attribute is still positive.

(Minimal example extracted from this code https://github.com/residualvm/residualvm/blob/master/backends/graphics/surfacesdl/surfacesdl-graphics.cpp)