Reviewer A comments, Reviewer B replies, B deletes comment, A deletes comment, A completes review — they see an error and cannot complete.
or
Reviewer A comments, Reviewer B replies with a draft comment, A deletes comment, A completes review — they see an error and cannot complete.

This can also happen when there is a draft review with a reply. I'm not sure how the UI allows that situation to occur, but a customer has seen it.

Update on 15/Jan/15:

• The error doesn't happen for comments on file that were deleted, but for General Comments only.
• With this error the reviewer is unable to complete the review, unless the workaround is applied.
• Other user (e.g. review's author) is still able to close the review (but not to complete it, as completing a review is an option only for reviewers).

Workaround

• Database - Modifying data

  Always back up your database before performing any modification to the database. If possible, try your modifications on a test server.

• Update the deleted comments directly in the database, so they will be back to the review.
• Stop Crucible before applying any changes to the database, and make sure that you have a database backup.
• Use the following SQL command to retrieve the comments affected by this problem:

  select * from cru_comment c
  where cru_deleted = true and exists(
    select * from cru_comment b where c.cru_comment_id = b.cru_reply_to_comment_id)
  

• Update the comments by "restoring them" (setting the deleted flag to false):

  update cru_comment set cru_deleted = false 
    where cru_comment_id in (<cru_comment_ids>); 
  

  Where <cru_comment_ids> is the list of the ids returned by the first SQL command.