Calling ComposerAutoloaderInit::getLoader twice when a package requires a .php file containing functions, lead to the functions to be declared twice, and cause an error.
In my case, using behat + symfony2extension + assetic, the error that occured:
PHP Fatal error: Cannot redeclare assetic_init() (previously declared in vendor/kriswallsmith/assetic/src/functions.php:20) in /vendor/kriswallsmith/assetic/src/functions.php on line 26
Fatal error: Cannot redeclare assetic_init() (previously declared in /vendor/kriswallsmith/assetic/src/functions.php:20) in /vendor/kriswallsmith/assetic/src/functions.php on line 26