  We examine the accuracy of the growth equation $\ddot{\delta} +
2H\dot{\delta} - 4\pi G\rho\delta = 0$, which is ubiquitous in the cosmological
literature, in the context of the Newtonian gauge. By comparing the growth
predicted by this equation to a numerical solution of the linearized Einstein
equations in the $\Lambda$CDM scenario, we show that while this equation is a
reliable approximation on small scales ($k\gtrsim $h Mpc$^{-1}$), it can be
disastrously inaccurate ($\sim 10^4% $) on larger scales in this gauge. We
propose a modified version of the growth equation for the Newtonian gauge,
which while preserving the simplicity of the original equation, provides
considerably more accurate results. We examine the implications of the failure
of the growth equation on a few recent studies, aimed at discriminating general
relativity from modified gravity, which use this equation as a starting point.
We show that while the results of these studies are valid on small scales, they
are not reliable on large scales or high redshifts, if one works in the
Newtonian gauge. Finally, we discuss the growth equation in the synchronous
gauge and show that the corrections to the Poisson equation are exactly
equivalent to the difference between the overdensities in the synchronous and
Newtonian gauges.
