//////////
Credit for all of these goes to "zu straff"

https://steamhost.cn/steamcommunity_com/games/648120/Avatar/List
https://steamhost.cn/steamcommunity_com/games/633360/Avatar/List
https://steamhost.cn/steamcommunity_com/games/628560/Avatar/List
https://steamhost.cn/steamcommunity_com/games/647000/Avatar/List
https://steamhost.cn/steamcommunity_com/games/630280/Avatar/List
https://steamhost.cn/steamcommunity_com/games/homewars/Avatar/List
//////////

//////////
Credit for all of these goes to a blank name

https://steamhost.cn/steamcommunity_com/games/fol/Avatar/Preview/0
https://steamhost.cn/steamcommunity_com/games/360400/Avatar/Preview/0
https://steamhost.cn/steamcommunity_com/games/569640/Avatar/Preview/0
https://steamhost.cn/steamcommunity_com/games/629210/Avatar/Preview/0
//////////

Currently, the only confirmed working 404 flags everywhere (including user profiles) are:

Kosovo

////////// Everything below this line only displays the flag on groups, user profiles show without any picture

Bonaire, Sint Eustatius and Saba
Isle of man
Guernsey
Jersey
Curacao
Sint Maarten (Dutch part)

- https://steamhost.cn/steamcommunity_com/market/listings/753/214730-%3Aranger%3A
All "Rogue Stormers" emoticons - Market link - Credit to "Cookie Maze"

Anything goes

If you know any bug / exploit that wasn't shared here yet feel free to create a new discussion!

All "Who's Your Daddy" achievements
"Two Hours Of Shower" from "Shower with your Dad simulator"
A small chunk of "Running Through Russia" achievements
A small chunk of "Killing Floor 2" achievements
"Test Name" from "Stranded Deep" - Credit to "zu straff"

"NEW_ACHIEVEMENT_NAME_0_4" from "Spacewar" is a special case because instead of displaying a broken image, it displays nothing at all. It is only obtainable via Steam Achievement Manager, both because you cannot unlock it from regular play, and because the game isn't on Steam.