ERROR: While executing gem ... (Errno::EINVAL)
Invalid argument - D:/expwebJunit/www/expweb/expdevtools/ruby/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mini_magick-3.6.0/test/files/special! "chars'.gif

解决方案：I ran into same problem and here was my solution to work around it. The problem seems to be in the new 3.6.0 version， so back out the new version and install the previous version.  gem uninstall mini_magick   gem install mini_magick -v 3.5.0    After installing 3.5.0 mini_magick gem， rerun the watir gem install. It should find the mini_magick gem already installed and skip over it and install the remaining gems successfully. Good luck.          Actually， this doesn't solve my problems， after installed mini_magick version 3.5.0， I still get the same error when I execute bundle install. I checked the Gemfile.lock file which generated few days ago and found the version for mini_magick is 3.5.0， then i copied Gemfile.lock to Gemfile directory and then executed bundle install， it's successful.

以后知道了，如果安装某个gem包时，在安装其依赖包时报错无法继续安装，则先卸载该依赖包，重新安装一个其他版本的尝试一下