synchronize hashmap lookups

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

@@ -40,8 +40,10 @@ public class LongHashtable<V> extends LongHash
     }
 
     public V get(long key) {
+        synchronized(this) {
             return containsKey(key) ? (V) cache.value : null;
         }
+    }
 
     public boolean containsKey(long key) {
         if (cache != null && cache.key == key)