I have also discovered this, but also found one example that is re-creatable.
My mobile phone is great for taking photo's but terrible for uploading pictures, it runs out of memory and is unable to load.
Showing me this message wipes my draft copy - its my phone that error's, so not sure why it does this. I now upload images to photo's/files and link from there instead. Taken me ages to put 2+2 together.
Hope this helps to contribute to a solution.
Having a "Save draft" button would solve all the random weird behaviour observed by differing users.