diff --git a/youtube_dl_webui/core.py b/youtube_dl_webui/core.py
index f544b1a..5544947 100644
--- a/youtube_dl_webui/core.py
+++ b/youtube_dl_webui/core.py
@@ -186,6 +186,7 @@ def event_batch(cls, svr, event, data, arg):
 class WorkMsgDispatcher(object):
 
     _task_mgr = None
+    logger = logging.getLogger('ydl_webui')
 
     @classmethod
     def init(cls, task_mgr):
@@ -212,10 +213,13 @@ def event_fatal(cls, svr, event, data, arg):
     @classmethod
     def event_progress(cls, svr, event, data, arg):
         tid, data = data['tid'], data['data']
-        cls._task_mgr.progress_update(tid, data)
-
-        if data['status'] == 'finished':
-            cls._task_mgr.finish_task(tid)
+        try:
+            cls._task_mgr.progress_update(tid, data)
+        except TaskInexistenceError:
+            cls.logger.error('Cannot update progress, task does not exist')
+        else:
+            if data['status'] == 'finished':
+                cls._task_mgr.finish_task(tid)
 
 
 def load_conf_from_file(cmd_args):
diff --git a/youtube_dl_webui/msg.py b/youtube_dl_webui/msg.py
index caba0ba..2ca3650 100644
--- a/youtube_dl_webui/msg.py
+++ b/youtube_dl_webui/msg.py
@@ -50,9 +50,11 @@ def __init__(self):
     def new_cli(self, cli_name=None):
         uuid = None
         if cli_name is not None:
+            # For named client, we create unique queue for communicating with server
             uuid = cli_name
             cli = CliMsg(cli_name, Queue(), self._svrQ)
         else:
+            # Anonymous client is a client who needn't to talk to the server.
             uuid = new_uuid()
             cli = CliMsg(uuid, None, self._svrQ)
 
@@ -61,6 +63,10 @@ def new_cli(self, cli_name=None):
         return cli
 
     def reg_event(self, event, cb_func, arg=None):
+        # callback functions should have the signature of callback(svr, event, data, arg)
+        #
+        # svr is an instance of SrvMsg class, so the callback can directly send
+        # mssages via svr to its corresponding client.
         self._evnt_cb_dict[event] = (cb_func, arg)
 
     def run(self):
diff --git a/youtube_dl_webui/task.py b/youtube_dl_webui/task.py
index b9e0a3c..218bb97 100644
--- a/youtube_dl_webui/task.py
+++ b/youtube_dl_webui/task.py
@@ -149,7 +149,7 @@ def start_task(self, tid, ignore_state=False):
             if status['state'] == state_index['finished']:
                 raise TaskError('Task is finished')
 
-            task = Task(tid, self._msg_cli, ydl_opts=ydl_opts, info=info, 
+            task = Task(tid, self._msg_cli, ydl_opts=ydl_opts, info=info,
                         status=status, log_size=self._conf['general']['log_size'])
             self._tasks_dict[tid] = task
 
@@ -279,5 +279,10 @@ def launch_unfinished(self):
         tid_list = self._db.launch_unfinished()
 
         for t in tid_list:
-            self.start_task(t)
+            try:
+                self.start_task(t)
+            except TaskError as e:
+                self.logger.warn("Task %s is in downloading or finished state", tid)
+            except TaskInexistenceError:
+                self.logger.error('Task does not exist')