Since December 8th, upon an attempt to add a recipient to a list, we get a "400 Bad Request" response from Constant Contact (using the API).
The problem does not seem to be happening for all recipients on a list. It may happen for some partial number of entire list. As a result, we assume that we never sent an email to these people as there were not added to a list.
Logging onto the Constant Contact website, we figured out that even though we get this error, the users are still added to the list AND sent an email.