Who's asking: Me
Well, yes and no. Vitamin C is a water-soluble vitamin, so the body excretes what it doesn't use; nevertheless, an acute overdose can cause various stomach and intestinal discomforts.
Because Vitamin C affects the absorption of other vitamins and minerals, chronic overdoses of Vitamin C can deplete the body of stored calcium and copper, and cause over-absorption of a particular type of iron. Pregnant women who take massive doses of Vitamin C can actually cause "rebound scurvy" in their babies, once the babies are born and suddenly deprived of those vitamin levels.
All that said, however, the four doses of Emergen-C I took yesterday (a total of 4,000 mg, vastly above the recommended daily allowance of 75 mg) have brought me back to the land of the living. The Theraflu might have helped, too, or maybe the cold has just run its course; but at least today I've been able to keep my eyes open and get a couple of things done. Hurray for Vitamin C!