Probably none. While overdosing any vitamin is never recommended, vitamin c is readily excreted in the urine so any excess Ascorbic Acid (vitamin c) doesn't usually build up in the body or cause long-term problems.
Mineral deficiencies. Mega dose vitamin c can actually be dangerous. It is true that vitamin c is water soluble & you will excrete any vitamin c that you don't use. However, in order to excrete vitamin c in your urine, you must also excrete minerals as well. Therefore, excess vitamin c can lead to mineral deficiencies, especially a copper deficiency. Be very cautious in megadosing any vitamin or mineral or amino acid.