The map +forest small_house starts at 2nd floor as the "ground" level, but its 1st floor is a pitch black featureless void.

You can't go down to that level and it doesn't affect gameplay as far as I've seen, but it's the only map to have such a blank level, so I assumed it's a minor bug.

log file:
<source>
------- Loading game.dll -------
2015/03/18 00:13:14 not found at '/usr/local/lib/'
2015/03/18 00:13:14 not found at 'C:\Users\USER\AppData\Roaming\UFOAI/2.6-dev/base'
2015/03/18 00:13:14 found at './base'
2015/03/18 00:13:14 Map info - map type: RMA
2015/03/18 00:13:14 Map info - map name: +forest
2015/03/18 00:13:14 Map info - assembly name: small_house
2015/03/18 00:13:14 SV_ParseAssembly: warning - cvar 'rm_ufo' value doesn't seem to be a valid tile id '' - set to default '+craft_crash_bomber'
2015/03/18 00:13:14 Map info - tiles used: -forest/fr_ +small_house +craft_drop_firebird +nature_12 +nature_21 +nature_12 +nature_12 +nature_33 +nature_11 +nature_30 +nature_30 +nature_11 +nature_12 +nature_11 +nature_02 +nature_11 +nature_20 +nature_20 +nature_20
2015/03/18 00:13:14 Map info - tiles pos: -32 -32 0 0 24 0 -32 -8 0 32 24 0 24 32 0 24 24 0 16 -32 0 -32 0 0 16 8 0 16 -8 0 16 -8 0 16 8 0 8 16 0 -16 -8 0 -24 0 0 -32 8 0 -32 24 0 -16 24 0
2015/03/18 00:13:14 Map info - tiles count: 18
2015/03/18 00:13:23 Rerouted 1488 cols for RMA in 7.0s
2015/03/18 00:13:23 checksum for the map '+forest': 4250714369
2015/03/18 00:13:23 ufo script checksum 2998464025
2015/03/18 00:13:23 -------------------------------------
2015/03/18 00:13:23 Connecting to localhost...
2015/03/18 00:13:23 ==== ShutdownGame ====
2015/03/18 00:13:23 Used inventory slots in game on shutdown: 0
2015/03/18 00:13:23 ==== InitGame ====
2015/03/18 00:13:23 Spawned ai player for team 0 with entnum 86 (b c)
2015/03/18 00:13:23 Spawned ai player for team 0 with entnum 87 (a c)
2015/03/18 00:13:23 Spawned ai player for team 0 with entnum 88 (a c)
2015/03/18 00:13:23 Spawned ai player for team 0 with entnum 89 (a c)
2015/03/18 00:13:23 Spawned ai player for team 0 with entnum 90 (a c)
2015/03/18 00:13:23 Spawned ai player for team 0 with entnum 91 (a c)
2015/03/18 00:13:23 Spawned ai player for team 0 with entnum 92 (a c)
2015/03/18 00:13:23 Spawned ai player for team 0 with entnum 93 (a c)
2015/03/18 00:13:23 Created AI player (team 0)
2015/03/18 00:13:23 Spawned ai player for team 7 with entnum 94 (HN/R 010.1100)
2015/03/18 00:13:23 Spawned ai player for team 7 with entnum 95 (HN/Z 001.1100)
2015/03/18 00:13:23 Spawned ai player for team 7 with entnum 96 (Kronuk Tkr Hruk)
2015/03/18 00:13:23 Spawned ai player for team 7 with entnum 97 (HN/U 010.1101)
2015/03/18 00:13:23 Created AI player (team 7)
2015/03/18 00:13:23 Used inventory slots after ai spawn: 0
2015/03/18 00:13:23 connection attempt from loopback connection
2015/03/18 00:13:28 load material file: 'materials/forest.mat'
2015/03/18 00:13:28 World model: 13796 triangles
2015/03/18 00:13:28 Planting grass ...
2015/03/18 00:13:28 Total grassy area is 0 units (0 cells)
2015/03/18 00:13:28 Planted 0 clumps of grass
2015/03/18 00:13:33 Starting the game...
2015/03/18 00:13:33 Alex has joined team 1
2015/03/18 00:13:33 Music: track changed from van_geoscape to TheThings-Slippycurb.
2015/03/18 00:13:33 Rerouted for 0 Edicts in 0.00s
2015/03/18 00:13:40 Used inventory slots: 0
2015/03/18 00:13:40 Used inventory slots client Alex spawn: 0
2015/03/18 00:13:40 (player 0) It's team 1's turn!
2015/03/18 00:13:40 Rerouted for 1 LEs in 0.00s
2015/03/18 00:13:40 Alex has taken control over team 1.
2015/03/18 00:13:49 radar forest/fr_small_house -32x-32
2015/03/18 00:13:49 radar forest/fr_craft_drop_firebird 0x24
2015/03/18 00:13:49 radar forest/fr_nature_12 -32x-8
2015/03/18 00:13:49 radar forest/fr_nature_21 32x24
2015/03/18 00:13:49 radar forest/fr_nature_12 24x32
2015/03/18 00:13:49 radar forest/fr_nature_12 24x24
2015/03/18 00:13:49 radar forest/fr_nature_33 16x-32
2015/03/18 00:13:49 radar forest/fr_nature_11 -32x0
2015/03/18 00:13:49 radar forest/fr_nature_30 16x8
2015/03/18 00:13:49 radar forest/fr_nature_30 16x-8
2015/03/18 00:13:49 radar forest/fr_nature_11 16x-8
2015/03/18 00:13:49 radar forest/fr_nature_12 16x8
2015/03/18 00:13:49 radar forest/fr_nature_11 8x16
2015/03/18 00:13:49 radar forest/fr_nature_02 -16x-8
2015/03/18 00:13:49 radar forest/fr_nature_11 -24x0
2015/03/18 00:13:49 radar forest/fr_nature_20 -32x8
2015/03/18 00:13:49 radar forest/fr_nature_20 -32x24
2015/03/18 00:13:49 radar forest/fr_nature_20 -16x24
2015/03/18 00:13:49 No radar images for map: 'forest/fr_small_house'
2015/03/18 00:14:02 [STATS] Alex (Hans Herrmann ROC) kills alien (HN/R 010.1100) with Aimed Shot of rpg_ammo (entnum: 94)
2015/03/18 00:21:26 Music: track changed from TheThings-Slippycurb to space.
2015/03/18 00:22:56 Music: track changed from space to ufo1.
2015/03/18 00:25:11 Music: track changed from ufo1 to PsymongR3.
2015/03/18 00:29:33 Music: track changed from PsymongR3 to PsymongO1.
2015/03/18 00:32:04 Music: track changed from PsymongO1 to PsymongN1.
2015/03/18 00:36:58 Music: track changed from PsymongN1 to Crystan-Battlescape05.
2015/03/18 00:42:10 Music: track changed from Crystan-Battlescape05 to Crystan-Battlescape02.
2015/03/18 00:47:35 Music: track changed from Crystan-Battlescape02 to PsymongO5.
2015/03/18 00:53:45 Music: track changed from PsymongO5 to PsymongR1.
2015/03/18 01:01:44 Music: track changed from PsymongR1 to Crystan-Battlescape03.
2015/03/18 01:05:54 Music: track changed from Crystan-Battlescape03 to van_mission2.
2015/03/18 01:10:50 Music: track changed from van_mission2 to PsymongN2.
2015/03/18 01:17:38 Music: track changed from PsymongN2 to PsymongO3.
2015/03/18 01:18:41 Taking JPG screenshot (at 75% quality)...
2015/03/18 01:18:41 Wrote scrnshot/ufo00.jpg to C:\Users\USER\AppData\Roaming\UFOAI/2.6-dev/base
2015/03/18 01:19:34 Taking JPG screenshot (at 75% quality)...
2015/03/18 01:19:34 Wrote scrnshot/ufo01.jpg to C:\Users\USER\AppData\Roaming\UFOAI/2.6-dev/base
2015/03/18 01:21:59 Music: track changed from PsymongO3 to PsymongR5.
2015/03/18 01:23:03 Music: track changed from PsymongR5 to PsymongN1.
</source>