With 2.5.1, the following problems arose:
- Could not assign a post to a user with a space in his username.
- Comma inserted when there were only two authors.
- 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.