If you try to login as another user and this user uses a language diferent than yours as can be English and Catalan. a message is shown beacause string_for_js detects $string['cancel'] differs between before and after of logging as.

Coding error detected, it must be fixed by a programmer: Attempt to re-define already required string 'cancel' from lang file 'moodle'. Did you already ask for it with a different $a? Cancel !== Cancel- la
Stack trace:

line 837 of /lib/outputrequirementslib.php: coding_exception thrown
line 299 of /lib/outputrequirementslib.php: call to page_requirements_manager->string_for_js()
line 1076 of /lib/outputrequirementslib.php: call to page_requirements_manager->init_requirements_data()
line 339 of /lib/outputrenderers.php: call to page_requirements_manager->get_head_code()
line 34 of /theme/atenea2012/layout/default.php: call to core_renderer->standard_head_html()
line 685 of /lib/outputrenderers.php: call to include()
line 637 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line ? of unknownfile: call to core_renderer->header()
line 1353 of /lib/setuplib.php: call to call_user_func_array()
line 2225 of /lib/weblib.php: call to bootstrap_renderer->__call()
line 2225 of /lib/weblib.php: call to bootstrap_renderer->header()
line 75 of /course/loginas.php: call to notice()