Sometimes a particular texture gets corrupted for random reasons (either read incorrectly from disk, or decompressed incorrectly, or copied with some error into the video card memory) - a single incorrect bit could cause some parts of a texture to be decoded incorrectly, depending on the compression format used for those textures.
It could be something as basic (but extremely rare) as your memory sticks having 1 bit randomly corrupted, while that area of the ram sticks held the texture before it got uploaded to the game.
Furmark USED to be dangerous because it loaded a video card with so much detail and information to be rendered that it causes the video card to consume way too much power, more than what the power converters on the video card were designed for. So those voltage regulators could overheat or could become damaged.
They have added checks in the drivers which detect when Furmark or a Furmark like application is started, and the drivers pay attention to how much power the video cards consume and they're more careful these days and put breaks if the video cards consume too much power.
So there's almost no risk of damaging a video card with benchmarks these days, on modern video cards.