This problem has been bugging me for some time now and I haven't been able to find this issue online anywhere else.
My Drupal 7 website has a very simple photo gallery, using a custom node type called gallery with image upload fields.
One or two of these nodes (but not all) have started behaving very strangely. When I upload new images, the files appear to upload successfully and the thumbnails appear, but immediately I get a message directly above the field on the edit page saying "Gallery (node title) has been updated." Normally this message would only appear after submitting changes using the save button.
When I click 'Save', I get the following message:
"The content on this page has either been modified by another user, or you have already submitted modifications using this form. As a result, your changes cannot be saved."
At this point, the node unpublishes itself, and the changes I make are not saved. The only way to publish the node again is by using the /admin/content interface.
I have noticed that nodes that have many images are the problematic ones. One such node has 160 images.
This behavior only happens with the gallery node type. I have tried creating a new node type with the same fields and recreating the node from scratch, but the same thing happens. I have tried using the image widget and the multiupload widget.