I ran into this issue for someone this week when cleearing out issues from Google webmaster tools. specifically I was trying to remove the problem of duplicate descriptions for web pages from their blog posts. here’s what All-in-One-SEO has for the meta description setting:

Here’s what I wanted it to do:

But what it did was put add this text to the beginning of my meta description tag!

I tried several other combinations, no luck. so I contacted the plugin developer and said:

Referring to this thread:
I have a client using this plugin, the free version, and I cannot get the meta description in posts to display the %post_title%. I was wondering if this bug was fixed in the pro version.

In their second response I gleened the following solution! Put this as your description:

And Viola! My post title, pulled in using the variable “%wp_title%” is included in my meta description. Thanks Semper Fi Web Design, great plugin and valueble support responses!