When stock and forex markets are closed, there is always a possibility, on the net opening, that the price will gap. 'Gap' means that it will open at a substantially higher or lower price than the previous close. When that happens, traders that have a protective stop loss that falls within that gap will not have the protection of a loss as the market never touches that point. The result is the trader will continue to lose until the broker issues a margin call and if the trader an not meet the call, his position will be bought/closed out by the broker.

I don't know why it would make any different if it were a 2 day or 3 day weekend.
Stok markets close on a daily basis, so there is always a possibility of any particular stock to gap. The forex market is only closed over the weekend, otherwise trades continuously.

http://www.forexfactory.com/showthread.php?t=234477