[Plugin: Co-Authors Plus] Worked fine before upgrade. Now tons of bugs.

With 2.5.1, the following problems arose:

1. Could not assign a post to a user with a space in his username.
2. Comma inserted when there were only two authors.
3. My guest authors defaulted to the account they were posted with (http://www.wpbeginner.com/wp-tutorials/how-to-rewrite-guest-author-name-with-custom-fields-in-wordpress/)

Thankfully, it was simple matter to downgrade to 2.1.1 and have everything work again.

http://wordpress.org/extend/plugins/co-authors-plus/