  It is shown that small distortions on the boundaries are amplified in the
core of a magnetized plasma if the system is close to marginal stability for
the ideal magnetohydrodynamic interchange mode. It is also shown that such
marginal systems can be nonlinearly unstable. The combination of boundary
amplification and nonlinearity is shown to result in a nonlinear instability.
The induced instability is highly sensitive to the boundary in that, if the
fractional deviation from marginality is a small parameter $b$, the system can
go unstable from fractional boundary distortions of $\mathcal{O}(b^{3/2})$.
