Update FAQ

Closes #3759.

readthedocs/quick-references/faq.rst

@@ -197,6 +197,32 @@ the most common patterns seem to be:
   multiple places, login and use a different session for each place you need.
 
 
+What does "Server replied with a wrong session ID" mean?
+========================================================
+
+This is a security feature from Telethon that cannot be disabled and is
+meant to protect you against unwanted session reuse.
+
+When this message is reported as a "bug", the most common patterns seem to be:
+
+* The proxy you're using may be interfering somehow.
+* The Telethon session is being used or has been used from somewhere else.
+  Make sure that you created the session from Telethon, and are not using the
+  same session anywhere else. If you need to use the same account from
+  multiple places, login and use a different session for each place you need.
+* You may be using multiple connections to the Telegram server, which seems
+  to confuse Telegram.
+
+Most of the time it should be safe to ignore this warning. If the library
+still doesn't behave correctly, make sure to check if any of the above bullet
+points applies in your case and try to work around it.
+
+If the issue persists and there is a way to reliably reproduce this error,
+please add a comment with any additional details you can provide to
+`issue 3759`_, and perhaps some additional investigation can be done
+(but it's unlikely, as Telegram *is* sending unexpected data).
+
+
 What does "Could not find a matching Constructor ID for the TLObject" mean?
 ===========================================================================
 
@@ -305,4 +331,5 @@ file and run that, or use the normal ``python`` interpreter.
 .. _logging: https://docs.python.org/3/library/logging.html
 .. _@SpamBot: https://t.me/SpamBot
 .. _issue 297: https://github.com/LonamiWebs/Telethon/issues/297
+.. _issue 3759: https://github.com/LonamiWebs/Telethon/issues/3759
 .. _quart_login.py: https://github.com/LonamiWebs/Telethon/tree/v1/telethon_examples#quart_loginpy

telethon/network/mtprotostate.py

@@ -176,7 +176,7 @@ class MTProtoState:
         reader = BinaryReader(body)
         reader.read_long()  # remote_salt
         if reader.read_long() != self.id:
-            raise SecurityError('Server replied with a wrong session ID')
+            raise SecurityError('Server replied with a wrong session ID (see FAQ for details)')
 
         remote_msg_id = reader.read_long()