diff --git a/tianshou/data/collector.py b/tianshou/data/collector.py
index 0066825..213b49d 100644
--- a/tianshou/data/collector.py
+++ b/tianshou/data/collector.py
@@ -364,6 +364,7 @@ class AsyncCollector(Collector):
         exploration_noise: bool = False,
     ) -> None:
         # assert env.is_async
+        warnings.warn("Using async setting may collect extra transitions into buffer.")
         super().__init__(policy, env, buffer, preprocess_fn, exploration_noise)
 
     def reset_env(self) -> None:
@@ -424,7 +425,6 @@ class AsyncCollector(Collector):
                 "Please specify at least one (either n_step or n_episode) "
                 "in AsyncCollector.collect()."
             )
-        warnings.warn("Using async setting may collect extra transitions into buffer.")
 
         ready_env_ids = self._ready_env_ids
 
diff --git a/tianshou/policy/modelfree/sac.py b/tianshou/policy/modelfree/sac.py
index fc89cf3..03469dc 100644
--- a/tianshou/policy/modelfree/sac.py
+++ b/tianshou/policy/modelfree/sac.py
@@ -174,6 +174,7 @@ class SACPolicy(DDPGPolicy):
 
         if self._is_auto_alpha:
             log_prob = obs_result.log_prob.detach() + self._target_entropy
+            # please take a look at issue #258 if you'd like to change this line
             alpha_loss = -(self._log_alpha * log_prob).mean()
             self._alpha_optim.zero_grad()
             alpha_loss.backward()