Let's get to the point. I'm just going to give you some of the common errors when joining or inside the server, not all because I'm not the smartest Gmod player alive. So, some of them are:

• Missing content/texture:
  Download the server and CSS content here: http://jot.sg/?page_id=72 .
  
  
• Missing Maps:
  Go to your Garry's mod option -> Multiplayer tab -> Find a drop menu that says, "When a game server tries to....." -> Choose "Allow all custom files from server."
  
  
• Connection error:
  This can be caused by server restart or DDoS attack. But if you feel that it's an attack please report it to us as soon as possible.
  
  Note: Could be a usual server segment error.
  
  
• Player Model Errors(Addition by Musica)
  Try to find the player model itself on the steam workshop, and download it manually from there. This should generally fix it.
  
  
• Missing Map .bsp(Addition by Musica)
  Locate your gmod folder, and search for the map that you are unable to load.
  Example: Missing Map is ttt_67thway, go to search, search in your gmod folder for 67thway. Delete everything related to it and reconnect to the server to redownload.
  
  
• The server you are trying to connect to is running an older version of the game. (Addition by unicorntart)
  Go to your Steam Games portal and right click on Garry's Mod -> Properties -> Betas Tab -> Select "None - Opt out of all beta programs". Restart Garry's Mod. If problem persists, it means that the server needs an update!
  
  
• The server you are trying to connect to is running an newer version of the game. (Addition by unicorntart)
  Update your Garry's Mod by restarting your Steam and open Gmod. Game will update to the latest stable release
  
  Works 90% of the time.

This topic will always be updated if there's any new common errors. Post a comment below if you know how to fix any other errors and we'll thank you very much. YEYESYSYESYES.

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-Fin-