I mentioned this in a general discussion thread but it will quickly get swamped where it is. Late-game there are 2 resources that are essential for units to be built but neither tech that reveals them are needed to actually research the techs that reveal them. All are at least 1 tier ahead of the unit tech but not all are in differing eras. Oil is revealed by Biology but the techs that reveal units that need oil: Flight - Triplanes & bombers Combined Arms - Tanks Radar - Heavy Bomber, fighters & B17 all do not need that tech. The same applies to aluminium revealed by Electricity. Mobile Tactics, which is much later, reveals Helicopter Gunships but Mobile Tactics doesn't need Electricity to be researched first. This is a serious mistake if this is by design. I understand the desire to have the separate lines... separate, but it is absurd to have a unit needing a resource researched when your civ has not researched that resource!