In my edition of AC (the one I bought here at GOG), when I have a combat unit with air drop ability, it can drop an unlimited number of times, from any square on the map, in a single turn. I was led to believe that airdrops have to start either in a base, or on an airbase.

Being able to air drop from anywhere has some pretty insane benefits. For one thing, I can send a couple of Drop Rovers all around the planet to open up any remaining supply pods. If one reveals a monolith, then that will end the unit's turn, but otherwise.... Although drop units take some damage when landing outside of a base, units never die from air dropping. The damage never goes below 95%.

So what say you? Bug, or intended benefit?

It is a known exploit. Am I correct if I say that you are using the right-click menu to make the airdrops instead of the hotkey? There are unofficial patches that fix it.

The only non-bug thing that might catch you off guard about airdrops is that after discovering Graviton Theory or if someone builds / captures the Space Elevator secret project, they can drop anywhere on Planet.

You are correct! I found THAT out the other day when I misplaced my mouse, and had to go all hotkey for a session. I'm like, "Hey, I remember that little parachute...." (I used to play the game a lot when it first came out, then I just came back to it this year.)

I just encountered this bug and it pretty much breaks the game (for a human player, the AI doesn't exploit it).

* Unlimited number of drops possible in 1 turn, because a drop costs 0 movement. Use drop units like mag-tubes to literally relocate them anywhere you want within your empire for 0 move cost (unlike mag tubes, units can effortlessly cross oceans this way too).
* Just drop to a city (regardless of where your unit begins or ends their turn) and the drop inflicts 0 damage. The city can be either friendly, OR an empty enemy city (see where this is going?)
* Units can still drop even if their turn has ended (the 'already moved' message is shown)... so drop next to an enemy, attack them (ending movement), then drop back into a friendly city with the same unit to heal (and redrop them some more to any city in your empire on the same turn... or drop-capture the enemy city if it is now empty!).
* Rolling Thunder: Spam-drop units next to an enemy city to capture it. Now use your newly captured city as a staging point to spam-drop against the next enemy city. Once you've broken the enemy's back (ie. captured their front line cities garrisoned by their best troops), their hinterland cities are usually garrisoned by much fewer/weaker troops, allowing 1 unit captures (ie. drop-attack-redrop into empty city, all with the same unit on the same turn). I've taken out entire enemy continents full of dozens of cities this way in only a few turns using a plague of drop units.
* Spam-capture enemy sub-sea cities in a surprise attack by dropping your specially outfitted scuba-para marines next to them on the coast. The only enemy cities that are immune to this are their sub-sea cities not built next to a coast... they will have to be taken the conventional way (oh dear).
* Elites that can not only drop but also move twice per turn are insane, giving you even more drop exploit options.

Any links to the mods that fix this bug? Of course I can simply stop abusing this bug, but what exactly are the 'normal' limitations on drops supposed to be?