ID:2181487
 
(See the best response by Nadrew.)
Code:
turf
icon = 'turf.dmi'
icon_state = ""
density = 0

turfs
icon_state = ""
estradas
terra_nova
icon = 'turf-estrada-terra-nova.dmi'
icon_state = ""
Top/icon_state = "top"
Right/icon_state = "right"
Bottom/icon_state = "bottom"
Left/icon_state = "left"
BorderIn1/icon_state = "1"
BorderIn2/icon_state = "2"
BorderIn3/icon_state = "3"
BorderIn4/icon_state = "4"
BorderOut5/icon_state = "5"
BorderOut6/icon_state = "6"
BorderOut7/icon_state = "7"
BorderOut8/icon_state = "8"


Problem description: Knows to inform me because this sometimes happens, at the moment of mapping everything is good, but then when it will run some places that one over the other of these problems of deleting everything that has down and is only the top, worse That takes place I use mountain and not of this problem.

Exemple:
What is the problem? The black edges? Try pressing like ctrl+shift when you're laying down your dirt and you should be fine. Also since your dirt layer doesn't have complete sides, you should probably up it's layer by 0.1 or something so it always appears above grass.

If you're manually mapping all of this, I suggest you look into: http://www.byond.com/developer/Foomer/13TileAutojoining

This library will save you tons of time since it automatically sets edges for icons. It also allows you to set particular states as well for when to turfs intersect, for example, when your dirt turfs meets your grass turfs.
Best response
To elaborate a bit, you can only really have a single turf on a tile at a time. When you place multiples down in the map editor, it'll overwrite the one that was placed there before it, and depending on your settings it may or may not add the previous turf as an underlay of the new one, giving the visual appearance of multiple turfs on a tile when reality it's just the top-most one.

In your case, it's overwriting and nothing else.
I found out here, thank you for your help.

It's because the bottom is like "" and inside the .dmi, has an unnamed name, needed by name.