I had a bizzare problem yesterday.
Normally I update the KLN94 from a 13-cycle database subscription but for complicated reasons I downloaded a one-off update from Honeywell.
Loading it into the KLN94 GPS, I found that not only would it not recognise any airport IDs but also all previous flight plans were deleted.
I loaded the previous cartridge back in (fortunately I still had it in my hands!) and that worked OK, except all the flight plans were still deleted.
I think what must have happened is that I downloaded something like the Pacific area database, and obviously none of the flight plans would have had any waypoints which would have featured in that.
The lesson is to always check, after a database update, that the GPS is still working....