Yes, it is possible. For every order (buy/sell), there should be a opposite order (sell in case of buy and vice versa) on the other side. Trading works on demand and supply matching theory. If the demand does not match supply or vice versa, or if the rate at which the order is placed does not find a match in the exchange, the trade will not take place.

Till the time the order does not get executed, order status will be shown as 'Pending'