  We discuss the behaviour of non-perturbative superpotentials in 4d N=1 type
II compactifications (and orientifolds thereof) near lines of marginal
stability, where the spectrum of contributing BPS D-brane instantons changes
discontinuously. The superpotential is nevertheless continuous, in agreement
with its holomorphic dependence on the closed string moduli. The microscopic
mechanism ensuring this continuity involves novel contributions to the
superpotential: As an instanton becomes unstable against decay to several
instantons, the latter provide a multi-instanton contribution which
reconstructs that of the single-instanton before decay. The process can be
understood as a non-perturbative lifting of additional fermion zero modes of an
instanton by interactions induced by other instantons. These effects provide
mechanisms via which instantons with U(1) symmetry can contribute to the
superpotential. We provide explicit examples of these effects for non-gauge
D-brane instantons, and for D-brane gauge instantons (where the motions in
moduli space can be interpreted as Higgsing, or Seiberg dualities).
