I once actually got so disorientated in thick fog on a route I knew well that I turned round and headed back home, following the pavements.
The classic whiteout is a hazard to skiers and mountaineers; movement is difficult without the usual visual cues and you tend to lose balance and fall over. You can walk or ski off a cornice without knowing it, which is why ski resorts have to plant marker poles and rope off dangerous areas. A ski touring guide once told me about a nightmare trip off piste where the fog was so thick that he resorted to tying his rucsac on a rope and dragging it behind him as a brake while making snowballs, throwing them ahead and skiing to them, so concerned was he about cliffs or crevasses.