What does "no display - blocked mbox" mean?

One of the mboxes on my site isn't working, and when I look at the response, there is an auto generated comment that says:

/* no display - blocked mbox */

What causes this, and what does it mean?

Other mboxes on the same page are working, it just this particular one, but I can't see anything different about the configuration, or the content we are trying to send.