Twitter Cards with Metatag module not working
If you´re using the 7.x-1.0-Beta4 version of metatag module (which is the most recent one at the time of writing this answer), you may want to apply the following patch:

http://drupal.org/node/1846978#comment-6764558

(for Unix-like systems only) For applying the patch, just go to a terminal, change the working directory to the directory of the metatag module and try this:

curl http://drupal.org/files/metatag-fix_theme_hook_mismatch-1846978-0.patch | patch -p1