Acountry with a command economy may have huge accumulations of goods that no one wants, while other products are in short supply. what does this situation seldom happen in market economies?

In a market economy the production is determined not by someone's decision (which can be wrong, and a wrong decision is the reason why there there are unwanted goods or a lack of wanted goods) but it is regulated by the supply and demand: if there is a need for a good, it will be produced, and if there is no need for something, its production will halt and there will not be an unwanted storage. 

In short, in a market economy, the economy itself regulates this. 

Afine-grained igneous rock forms as a result of slow cooling.