I was writing quite a long post and frequently using the 'Save & continue editing' button, which seems to be fine (as no errors popped-up, see issue #472). When using the 'Save' button to finish up an error (404) showed.

Steps to reproduce (without writing long posts :P ):

• Open an existing blog post
• Change title (so URL/slug changes)
• Click 'Save & continue editing'
• Now save again using 'Save & continue editing'

This actually makes sense, as the URL got changed along the way and the form submits to a URL/slug that is no longer recognized by the blog engine.