  We present results of numerical simulations of coupled Ginzburg-Landau
equations that describe parametrically excited waves. In one dimension we focus
on a new regime in which the Eckhaus sideband instability does not lead to an
overall change in the wavelength via the occurrence of a single phase slip but
instead leads to ``double phase slips''. They are characterized by the phase
slips occurring in sequential pairs, with the second phase slip quickly
following and negating the first. The resulting dynamics range from transient
excursions from a fixed point resembling those seen in excitable media, to
periodic solutions of varying complexity and chaotic solutions. In larger
systems we find in addition localized spatio-temporal chaos, where the solution
consists of a chaotic region with quiescent regions on each side. We explain
the localization using an effective phase diffusion equation which can be
viewed as arising from a homogenization of the chaotic state. In two dimensions
the double phase slips are replaced by fluctuating bound defect pairs.
