The aliendrone entity never despawns and can wander off map. If you attack a power crystal then run away leaving the drones there (and do not kill them) they will eventually begin to wander off the map and into ungenerated regions. Given enough hours (it can take 100+) they will reach the maximum map size border and cause this game crash:

Error Chunk.cpp:701: Trying to make chunk at unreasonable position [some big coordinate numbers]

If you have lost your save from this problem here is a Lua script that will fix it without having to remove the Fortress Ruins mod (it's awesome, who wants to lose it?) (however running this script will unfortunately disable achievements):

/c
for , surface in pairs(game.surfaces) do
local entities = surface.find_entities_filtered({name = "aliendrone"})
for , ent in pairs(entities) do
ent.die()
end
end

this bug is fixed in a other mod https://mods.factorio.com/mod/FortressRuinsFixAliendroneTimer as of what I've seen here