In a celebrated three-page-long paper in 1979, Aronson and Bénilan obtained a remarkable estimate on second-order derivatives for the solution of the porous media equation. Since its publication, the theory of porous medium flow has expanded relentlessly with applications including thermodynamics, gas flow, and groundwater flow, as well as ecological population dynamics. The purpose of this paper is to clarify the use of recent extensions of the Aronson and Bénilan estimate in L^p spaces and of some modifications and improvements, as well as to show certain limitations of their strategy.