There is an issue in WordPress v3.5 causing an error of in_admin() when loading the Tri.be Events community pages.

tribe-community-events.class.php

<?php

/**
 * There is an issue in WordPress v3.5 causing an error of in_admin() when loading the community pages.
 * Here is the fix:
 * 1. open lib/tribe-community-events.class.php (in your community events plugin folder)
 * 2. comment out the line add_action( 'wp', array( $this, 'maybeLoadAssets' ) );
 * 3. replace function init() with the one provided below
 */

// in WordPress v3.5 this is causing an error. So comment out.
// add_action( 'wp', array( $this, 'maybeLoadAssets' ) );

// replace the function init() method with the following code
public function init() {
  self::$instance = self::instance();
  self::loadTextDomain();
  self::maybeFlushRewriteRules();
  self::maybeLoadAssets();
}