  Local simulations of the magnetorotational instability (MRI) in accretion
disks can exhibit recurrent coherent structures called channel flows. The
formation and destruction of these structures may play a role in the
development and saturation of MRI-induced turbulence, and consequently help us
understand the time-dependent accretion behaviour of certain astrophysical
objects. Previous investigations have revealed that channel solutions are
attacked by various parasitic modes, foremost of which is an analogue of the
Kelvin-Helmholtz instability. We revisit these instabilities and show how they
relate to the classical instabilities of plasma physics, the kink and pinch
modes. However, we argue that in most cases channels emerge from developed
turbulence and are eventually destroyed by turbulent mixing, not by the
parasites. The exceptions are the clean isolated channels which appear in
systems near criticality or which emerge from low amplitude initial conditions.
These structures inevitably achieve large amplitudes and are only then
destroyed, giving rise to eruptive behaviour.
