Since Composer 1.7.0, running a script invokes the script with a forced --ansi parameter, which may case the invoked command to fail because it does not recognize this parameter.

  "scripts": {
    "test": "phpunit -c test"
}

composer test -- --coverage-clover=build/logs/clover.xml

> phpunit -c test '--coverage-clover=build/logs/clover.xml' '--ansi'
PHPUnit 5.7.27 by Sebastian Bergmann and contributors.
unrecognized option --ansi
Script phpunit -c test handling the test event returned with error code 1

See also the Travis build output.