On the night of June 26 UT when moving the 2.4″ longslit into the focal plane, the mask failed to insert into the focal plane despite the software thinking so.  When the instrument was opened, the grabber was found to be in the FPU but the mask was still in the cassette.  Further tests showed the same issue at the same telescope/instrument orientation (EL=40 deg., instrument rotation=238 deg.).  A temporary solution was found by removing all of the custom masks in slots 13-24 and placing the 2.4″ long slit mask into position 24, after which it was inserting into the FPU without issue.