As part of the re-design moving script know-how from the planner to the unit database (see previous news entry), apparently I've missed a few dependencies. This lead to a hand-ful of mission planning failures, and has now been fixed. Apologies!
I've just updated some 350 out of 6100 units with additional script references. The big thing I missed (but the mission planner noticed) is that you can use armed transport helicopters for gunship support when they are not tasked to transport infantry to the battlefield.
Not sure whether the mission planner is smart or I am stupid.