In early November 2010, Iridium 11 (originally 25577, 1998-074A, but currently labelled by Space-Track as 25578, 1998-074B), previously spare, was raised to the operational orbit, just a few seconds behind Iridium 23 (24906, 1997-043D). This suggested that Iridium 23 must have failed on station.
However, Iridium 23 appeared still to be under control and remains on station, so there may have been only a partial or temporary failure. The two satellites remained a few seconds apart.
In early August 2011, Iridium 11 was moved around the plane, evidently to take over from Iridium 26 (24903, 1997-043A).
This suggests that Iridium 26 must have failed on station, and also that Iridium 23 retains at least some functionality.
Orbital Plane 2 had no other spare satellite.
On 13 November, 2012, Iridium 94 (27374, 2002-005C), which had been migrating over the past year from orbital place 3, arrived at orbital plane 2, and was immediately raised to operational altitude to replace Iridium 23 (24906, 1997-043D) which had evidently failed, though retaining at least some functionality. Iridium 23 remains at operational altitude a few seconds behind Iridium 94