A white wine that was clear once but now is hazy or is hazier when it cold than when it is warm is suffering from protein haze. This is largely an aesthetic problem but one which winemakers try to eliminate. This article explains what home winemakers can do.
Reduced sulfur compounds (those formed in the absence of oxygen) are a perennial problem in winemaking because they usually smell very foul (rubber, swamp, cabbage, garlic). They also reduce a wines mouth feel and enhance bitterness.