  The evolution of weak discontinuity is investigated in the flat FRW universe
with a single scalar field and with multiple scalar fields. We consider both
massless scalar fields and scalar fields with exponential potentials. Then we
find that a new type of instability, i.e. kink instability develops in the flat
FRW universe with massless scalar fields. The kink instability develops with
scalar fields with considerably steep exponential potentials, while less steep
exponential potentials do not suffer from kink instability. In particular,
assisted inflation with multiple scalar fields does not suffer from kink
instability. The stability of general spherically symmetric self-similar
solutions is also discussed.
