returning if a thread is not alive should happen before any changes to

the list of workers, hence the return statement should be in the
synchronized block.

src/main/java/org/bukkit/craftbukkit/scheduler/CraftThreadManager.java

@@ -61,8 +61,8 @@ public class CraftThreadManager {
                     return craftWorker.isAlive();
                 }
             }
-        }
             // didn't find it, so it must have been removed
             return false;
         }
+    }
 }