When two solids are in relative sliding motion, the intervening layer separating the two surfaces (for example the boundary lubricant) is typically far from thermal equilibrium. With the help of a generic model reflecting the boundary lubricant, it will be shown that it is often not possible to characterize a sliding contact by means of a single effective temperature. The reason is that the probability distribution (PD) of microscopic variables differs in a characteristic fashion from equilibrium PDs. Non-equilibrium velocity PDs are not Gaussian but tend to be exponential, thus favoring rare events. Leaving dynamic equilibrium by non-uniform sliding conditions leads to yet additional effects, in particular to enhanced dissipation. This is shown in a model describing rubbing polymer brushes in good solvent conditions. Shortly after returning the sliding velocity, the brush interdigitation is distinctly larger than during steady-state sliding. Based on this observation, predictions can be made at what amplitude the loss is maximum for a given driving frequency.