A complete discussion of the "discontinuation effect" (sometimes called the rebound effect) was here, may still be here, otherwise I'll try to find it for the archives.
In a nut shell, the Bowel Tolerance forum - and the http://www.orthomed.com/titrate.htm
paper by Cathcart helps people determine what the right amount of vitamin C is for themselves. The answer is somewhere between 4000 mg and 20,000 mg, spread out 4 times throughout the day.
Most people live in a state of vitamin C starvation. A good analogy is the body's response to little water in a desert, the body begins to hold on to its water during periods of dehydration.
Vitamin C is handled in the same way. The body holds on to what little it can get. But, when the supply is ample - such as the case with most animals producing 24/7 - the body no longer needs to horde vitamin C and various enzyme systems begin that transform ascorbate into other substances.
If the supply is then suddenly cut off, these enzyme systems still transform C for 24 to 48 hours, and this is the discontinuation effect. It is not known to be serious, but a cold or congestion is likely.
Pauling recommended not stopping vitamin C even for a single day. I agree. But you can also lower the dosage slowly to avoid this effect.