I'm sharing this as it's an interesting defect we discovered on a client.  The solution is basically to remove Optimizely Labs Block Enhancements and upgrade to the latest version of CMS 12 as the Block Enhancements module has been deprecated in favour of core functionality improvements.

• Create and publish a new Page
• Add a block to a content area within the Page
• Publish the page.
  • Optimizely Labs Block Enhancements will publish the block too

At this point everything is happy

• Make a change to the block but do not publish it.
• Make a change to the page but do not publish it.
• Navigate to the full editor inteface for the block (not the quick editor)
• In the version widget, delete the draft for the block using the ellipse in the bottom left corner of the widget
  • 
  • This is an example of deciding we don't really want that change for that specific block anymore.
• Navigate back to the page and note that the page does not load in the editor interface

It would appear that as part of the page / block linked publishing life cycle, the content areas for the page contain a version specific reference to the block, specifically the draft version that has been deleted.  The Custom Content Area Loader that comes with Optimizely Labs Block Enhancements then errors trying to load the specific draft version of the block that no longer exists.

EPiServer.Core.ContentNotFoundException: 
  at EPiServer.Core.Internal.DefaultContentLoader.Get (EPiServer, Version=12.14.0.0, Culture=neutral, PublicKeyToken=8fe83dea738b45b7)
  at Advanced.CMS.ExternalReviews.DraftContentLoader.Get (Advanced.CMS.ExternalReviews, Version=1.3.3.0, Culture=neutral, PublicKeyToken=null)
  at EPiServer.Labs.BlockEnhancements.InlineBlocksEditing.CustomContentAreaLoader.Get (EPiServer.Labs.BlockEnhancements, Version=0.0.0.0, Culture=neutral, PublicKeyToken=8fe83dea738b45b7)

I've attempted to reproduce the same issue following the same steps in another CMS which has the External Reviews module but does not have the Block Enhancements module and this issue does not exist.

I don't expect a fix to be published as stated above ... just remove Block Enhancements module from your CMS and upgrade to the latest version :)