The very peaked pressure profiles in some pellet fuelled discharges with ion cyclotron resonance heating in the Joint European Torus (JET) are terminated by an abrupt flattening of the temperature profile. This appears to be the result of an instability with a very fast time-scale that is triggered when the safety factor q drops below 1.5. The ideal m/n = 3/2 infernal mode, which is excited only when q is very close to 1.5, may well be the responsible instability. Linear magnetohydrodynamic stability calculations, based on equilibria that model the relevant JET discharges as closely as possible, yield growth rates consistent with the rise time of the instability. Non-linear calculations initialized with the same equilibria exhibit plasma behaviour very similar to that seen experimentally: (1) a flattening of the pressure profile occurs, (2) a residual m = 3 structure is present after the flattening, and (3) the time-scale for the flattening is in reasonable agreement with experimental observations. It is thus concluded that the ideal infernal mode is a viable candidate for the instability.