Because the notion of "fairness" is highly subjective, free riding is usually only considered to be an economic "problem" when it leads to Pareto inefficiency.
One potential example of a free rider problem is National Defence: no person can be excluded from being defended by a nation's military and thus free riders may develop who refuse or avoid paying for being defended, but are still as well guarded as everyone else in the nation.
The problem is particularly important and troublesome when considering goods or resources access to which cannot be excluded. For more information, see tragedy of the commons.
See also TANSTAAFL, Public good.
With regard to a literal free ride, see also Conductor (transportation), Stowaway.References