  Optimised stellarators and other magnetic-confinement devices having the
property that the average magnetic curvature is favourable for all particle
orbits are called maximum-$J$ devices, and have recently been shown to be
immune to trapped-particle instabilities driven by the density gradient.
Gyrokinetic simulations reveal, however, that another instability can arise,
which is also associated with particle trapping but causes less transport than
typical trapped-electron modes. The nature of this instability is clarified
here. It is shown to be similar to the "ubiquitous mode" in tokamaks, and is
driven by ion free energy but requires trapped electrons to exist.
