Possible Player.dat fixes to address issues with switching between Vanilla and Bukkit or vice versa.

src/main/java/net/minecraft/server/Entity.java

@@ -980,7 +980,17 @@ public abstract class Entity {
         // CraftBukkit end
 
         // CraftBukkit Start - reset world
-        org.bukkit.World world = Bukkit.getServer().getWorld(nbttagcompound.getString("World"));
+        org.bukkit.World world = null;
+        if (this instanceof EntityPlayer) {
+            EntityPlayer entityPlayer = (EntityPlayer) this;
+            String worldName = nbttagcompound.getString("World");
+            if (worldName == "") {
+                world = (org.bukkit.World) ((CraftServer) Bukkit.getServer()).getServer().a(entityPlayer.dimension);
+            } else {
+                world = Bukkit.getServer().getWorld(worldName);
+            }
+        }
+
         a(world == null ? null : ((CraftWorld) world).getHandle());
         // CraftBukkit End
     }

src/main/java/net/minecraft/server/EntityHuman.java

@@ -341,7 +341,13 @@ public abstract class EntityHuman extends EntityLiving {
             this.a(true, true, false);
         }
 
-        this.spawnWorld = nbttagcompound.getString("SpawnWorld"); // CraftBukkit
+        // CraftBukkit - Start
+        this.spawnWorld = nbttagcompound.getString("SpawnWorld");
+        if (this.spawnWorld == "") {
+            CraftServer server = ((WorldServer) world).getServer();
+            this.spawnWorld = server.getWorlds().get(0).getName();
+        }
+        // CraftBukkit - End
 
         if (nbttagcompound.hasKey("SpawnX") && nbttagcompound.hasKey("SpawnY") && nbttagcompound.hasKey("SpawnZ")) {
             this.b = new ChunkCoordinates(nbttagcompound.e("SpawnX"), nbttagcompound.e("SpawnY"), nbttagcompound.e("SpawnZ"));