Thank you for the simple but useful plugin!
I'm here to inform you that I got some 404 after the CSS optimization.
The errors was such:
GET http://www.---.com/wp-content/themes/---/%20graphic/info.png 404 (Not Found)
So I removed all the spaces inside the url() rule and everything was ok.
PS: I have many css files in various folder. The error was returned from some css file into the theme root folder