  We investigate the observational consequences of the quintessence field
rolling to and oscillating near a minimum in its potential, "if" it happens
close to the present epoch (z<0.2). We show that in a class of models, the
oscillations lead to a rapid growth of the field fluctuations and the
gravitational potential on subhorizon scales. The growth in the gravitational
potential occurs on timescales << H^(1). This effect is present even when the
quintessence parameters are chosen to reproduce an expansion history consistent
with observations. For linearized fluctuations, we find that although the
gravitational potential power spectrum is enhanced in a scale-dependent manner,
the shape of the dark matter/galaxy power spectrum is not significantly
affected. We find that the best constraints on such a transition in the
quintessence field is provided via the integrated Sachs-Wolfe (ISW) effect in
the CMB temperature power spectrum. Going beyond the linearized regime, the
quintessence field can fragment into large, localized, long lived excitations
(oscillons) with sizes comparable to galaxy clusters; this fragmentation could
provide additional observational constraints.
  Two quoted "signatures" of modified gravity are a scale-dependent growth of
the gravitational potential and a difference between the matter power spectrum
inferred from measurements of lensing and galaxy clustering. Here, both effects
are achieved by a minimally coupled scalar field in general relativity with a
canonical kinetic term.
