I just found a strange bug...
I increased the size of a map (10 tiles or so) and copy pasted soome parts. Everything looked fine,; I've been editing the scenario with the map for a couple months now, and tonight I just tried playing it without debug mode on. Guess what?
Some (most) of the area I added is not covered in black, as if it had already been explored. It starts the game already revealed, although stil covered in fog of war. Which parts of the map are covered and which are visible seems almost random. No land tiles are visible, it is mostly, but not only, ocean tiles. And this is just in the added section of the map.
So, no idea the cause of this, other than the use of the increase size tool. If you use it, make sure to test the map out without debug mode on.
I'm sure theres a way to fix it, if anyone else has had this problem before please tell.
Here is the rarely used groucho head