Players

I've mined/deconstructed thousands of trees but never found a map or a hidden enemy.

You must destroy the trees: shoot them, explode them, burn them, roll them down with your tank, etc. Poisoning trees won't work, however. Also note that fire and explosions will consume small and medium maps, only fragments of big maps will remain.

What enemies will be hidden in trees?

All kinds: the biters, spitters, and worms from the vanilla game as well as those created by other mods.

What size do the hidden enemies have?

The maximum size of worms depends on the evolution factor, the maximum size of biters and spitters on the pollution in the chunk. That means that shooting down a dead tree at the beginning of the game could awaken a small worm, but shooting down a living tree before you've got some machines running (or at a place outside of the pollution cloud) will reveal no enemy at all. The actual size of a hidden enemy is randomly chosen: it will never exceed the maximum size, but it may be smaller.

Modders

I've defined a unit/turret prototype. If you spawn it, you break my mod!

Just run this during script.on_init:

-- patterns: string or array of strings
if remote.interfaces["MAPS"] and remote.interfaces["MAPS"]["dont_spawn_these_units"] then
  remote.call("MAPS", "dont_spawn_these_units", patterns)
end

if remote.interfaces["MAPS"] and remote.interfaces["MAPS"]["dont_spawn_these_worms"] then
  remote.call("MAPS", "dont_spawn_these_worms", patterns)
end

A pattern may be plain text or contain regular expressions that work with string.match(). If you do use regular expressions, don't forget to escape special characters! If you have just one pattern, you may pass it on as a string for simplicity. If you have several, it's more performant to pass on an array of strings containing all patterns.

Entities from these mods will be ignored per default:

AAI Programmable Vehicles,
Creative Mod,
Kombat Drones, Mining Drones, Transport Drones,
Updated Construction Drones,
Will-o'-the-Wisps updated

Maps