  We address the problem of infrared singularities in the perturbation theory
for disordered interacting systems in $d\leq 2$. We show that a typical,
sufficiently large interacting system exhibits a linear instability in the spin
triplet channel. In a density-density channel, although stability is preserved,
a large number of soft modes is accumulated. These phenomena are responsible
for the instability of the weak-interacting fixed point. Although generic, the
unstable direction and soft modes are highly sample specific and can not be
effectively captured by conventional techniques based on an averaging
procedure. Rather, the instability is determined by the largest eigenvalues of
the polarization operator. We propose to employ the optimal fluctuation method
for evaluating the probability of such events.
