diff --git a/OpenRa.Game/Bridges.cs b/OpenRa.Game/Bridges.cs
index 939bc5f2b3..1d9ad28626 100644
--- a/OpenRa.Game/Bridges.cs
+++ b/OpenRa.Game/Bridges.cs
@@ -52,7 +52,7 @@ namespace OpenRa
 			{
 				var a = w.CreateActor("Bridge", new int2(ni, nj), null);
 				var br = a.traits.Get<Bridge>();
-				br.SetTiles(template, replacedTiles);
+				br.SetTiles(w, template, replacedTiles);
 			}
 		}
 
diff --git a/OpenRa.Game/Game.cs b/OpenRa.Game/Game.cs
index 71246c54cf..96e3f572fb 100644
--- a/OpenRa.Game/Game.cs
+++ b/OpenRa.Game/Game.cs
@@ -58,6 +58,7 @@ namespace OpenRa
 			FileSystem.UnmountTemporaryPackages();
 			Rules.LoadRules(mapName, usingAftermath);
 
+			world = null;	// trying to access the old world will NRE, rather than silently doing it wrong.
 			world = new World();
 			Game.world.ActorAdded += a => 
 			{ 
diff --git a/OpenRa.Game/PathFinder.cs b/OpenRa.Game/PathFinder.cs
index e27ca54284..aabf600d62 100644
--- a/OpenRa.Game/PathFinder.cs
+++ b/OpenRa.Game/PathFinder.cs
@@ -4,6 +4,7 @@ using System.Linq;
 using OpenRa.FileFormats;
 using OpenRa.Support;
 using OpenRa.Traits;
+using System.Diagnostics;
 
 namespace OpenRa
 {
@@ -143,9 +144,7 @@ namespace OpenRa
 			return ret;
 		}
 
-
-
-		[System.Diagnostics.Conditional( "SANITY_CHECKS" )]
+		[Conditional( "SANITY_CHECKS" )]
 		static void CheckSanePath( List<int2> path )
 		{
 			if( path.Count == 0 )
@@ -160,7 +159,7 @@ namespace OpenRa
 			}
 		}
 
-		[System.Diagnostics.Conditional("SANITY_CHECKS")]
+		[Conditional("SANITY_CHECKS")]
 		static void CheckSanePath2(List<int2> path, int2 src, int2 dest)
 		{
 			if (path.Count == 0)
diff --git a/OpenRa.Game/PathSearch.cs b/OpenRa.Game/PathSearch.cs
index e4db641604..30c79ae194 100755
--- a/OpenRa.Game/PathSearch.cs
+++ b/OpenRa.Game/PathSearch.cs
@@ -39,7 +39,12 @@ namespace OpenRa
 			var p = queue.Pop();
 			cellInfo[ p.Location.X, p.Location.Y ].Seen = true;
 
-			if (passableCost[(int)umt][p.Location.X, p.Location.Y] == float.PositiveInfinity)
+			var custom2 = Game.world.customTerrain[p.Location.X, p.Location.Y];
+			var thisCost = (custom2 != null) 
+				? custom2.GetCost(p.Location, umt) 
+				: passableCost[(int)umt][p.Location.X, p.Location.Y];
+
+			if (thisCost == float.PositiveInfinity) 
 				return p.Location;
 					
 			foreach( int2 d in Util.directions )
@@ -51,6 +56,7 @@ namespace OpenRa
 					continue;
 
 				var custom = Game.world.customTerrain[newHere.X, newHere.Y];
+				if (custom != null) throw new NotImplementedException();
 				var costHere = (custom != null) ? custom.GetCost(newHere, umt) : passableCost[(int)umt][newHere.X, newHere.Y];
 
 				if (costHere == float.PositiveInfinity)
diff --git a/OpenRa.Game/Traits/Bridge.cs b/OpenRa.Game/Traits/Bridge.cs
index 0ef1ff41f1..5cc841a203 100644
--- a/OpenRa.Game/Traits/Bridge.cs
+++ b/OpenRa.Game/Traits/Bridge.cs
@@ -32,17 +32,17 @@ namespace OpenRa.Traits
 				yield return new Renderable(t.Value, Game.CellSize * t.Key, PaletteType.Gold);
 		}
 
-		public void SetTiles(TileTemplate template, Dictionary<int2, int> replacedTiles)
+		public void SetTiles(World world, TileTemplate template, Dictionary<int2, int> replacedTiles)
 		{
 			Template = template;
 			Tiles = replacedTiles;
 
 			foreach (var t in replacedTiles.Keys)
-				Game.world.customTerrain[t.X, t.Y] = this;
+				world.customTerrain[t.X, t.Y] = this;
 
-			if (cachedTheater != Game.world.Map.Theater)
+			if (cachedTheater != world.Map.Theater)
 			{
-				cachedTheater = Game.world.Map.Theater;
+				cachedTheater = world.Map.Theater;
 				sprites = new Cache<TileReference, Sprite>(
 				x => SheetBuilder.Add(Game.world.TileSet.GetBytes(x),
 					new Size(Game.CellSize, Game.CellSize)));