For most products, quantity supplied (QS) and quantity demanded (QD) are in equilibrium most of the time. There may be slight changes in price due to changes in QS or QD, but these changes will not typically be very large. It is unlikely that people will ever be unable to buy the amount of these products that they want.
The main kinds of things for which QS and QD do not match up are things that are very rare. These are things like tickets to certain sporting events or brand-new items like a new iPad when it first comes on sale. In these cases, QD is typically much higher than QS. The two examples given here have different reasons behind the discrepancies. For sporting events, there is simply no way to expand the supply. A stadium has only a certain number of seats and more cannot easily be added for hugely popular events such as the Super Bowl. For the iPad, it is possible that Apple could produce a huge number of the devices and hold them in storage so that there would be plenty available when they are first made available. However, this is not in the firm’s interest. It is more helpful for the firm to create a situation in which there is a mad scramble for the devices because it makes it look as though the devices are a must-have.