Oh, got it.
Find:
Code:
$challenges = $db->query_read("SELECT arcade_challenges.*, touser.username AS tousername, fromuser.username AS fromusername, arcade_games.miniimage, arcade_games.title FROM " . TABLE_PREFIX . "arcade_challenges
Replace with
Code:
$challenges = $db->query_read("SELECT arcade_challenges.*, touser.username AS tousername, fromuser.username AS fromusername, arcade_games.miniimage, arcade_games.title FROM " . TABLE_PREFIX . "arcade_challenges AS arcade_challenges