How is it possible for users to submit a node without a required image field? I am using Drupal 7 and I have defined a custom content type, Picture, which has only one required field, an image field.  I am using the default core image field with the Manual Crop module.  The number of images to be added is limited to one.  The field is marked as required.

If it matters, the title is not required because it is automatically generated by the Automatic Node Titles module.

I created this content type to allow users to upload their pictures to the site.  However, sometimes users are somehow able to create nodes of this content type that do not have any image file attached.  Since the image field is marked as required, I do not understand how Drupal can permit the creation of the node without such a file attached.  However, when I click edit, there is no file in the image file dialog box.

I have tried to debug this problem, but I am unable to create a node without an image file myself.  What kind of situation can cause this problem, or how can I debug it?
A contributed module was to blame-- the Manual Crop module.  Thankfully, the issue is already fixed.