Early in the list of custom maps, there is a series of maps that act as tutorials for the units. In them, you have to connect to the unit techs and keys to unlock them for use in the mission and in other ones.
Well, here's a good question: was it intended that collecting the key without collecting the unit tech would allow use of the unit in other levels while not in the level they are introduced in? ???
Put it this way: if you collect a key, the intro'ed unit can be used in other missions even if you haven't unlocked it (collected the unit tech) in the mission you find it in.
It wouldn't really matter, as keys should only apply if the map is completed and to complete the map you have to collect both the key and the tech. At least this was true for the story and I've seen multiple reports saying that only collecting the key also works. I check for new collected keys each time the map is loaded.
If only collecting the keys works, can that be reverted?