Comment 1 for bug 476403

Cookie check is likely completely incompatible with Pressflow's backport of lazy session creation from Drupal 7.

Pressflow prevents the creation of sessions for anonymous users until really necessary. This is an important performance enhancement across the board, and also is key in our use of Varnish, which bases its decision as to serving a cached page or not on the presence of a session cookie.

This plus the module's use of the validate hook will consistently mean that initial validation will fail because there's no cookie prior to the first form submit.

Unfortunately, these systems are fundamentally at odds, and I don't believe you can use the cookie_check with Pressflow or Varnish, and thus it's going to be problematic with Mercury.