After upgrading an application to Symfony 2.3.x I stumbled across this exception just the other day.
The documentation explains in detail why this can be the case and how to solve the issue.
However, it didn't work for me although ICU 4.8.1 is installed on our server which phpinfo() clearly stated but php -i | grep ICU did not.
And this was a hint to the solution: CLI and Apache use different php.ini files thus I had to provide the correct ini when invoking composer:
php -c /path/to/php.ini composer.phar install.

Woot! Look what I just found in my inbox:

SensioLabs is glad to inform you that you've been retroactively graduated the SensioLabs Symfony2 Certified Advanced Developer after you took the Symfony Certification exam between 2012 and 2013.

That makes me one happy guy!

Be a sportsman and take the WebDev Quiz. Now. Do it. Or let your apprentice take it to see if he's got what it takes ;)

Since using Git for version control and deployment those pesky hidden files on OSX are not a real problem anymore as you can .gitignore them. But sometimes they are so here's some pest control. Enter the directory in terminal and fire:

find . -name "._*" -print0 | xargs -0 rm -rf