MultiValueDictKeyError: "'body-0-deleted'"

We keep getting this issue:

MultiValueDictKeyError: "'body-0-deleted'"
  File "django/core/handlers/base.py", line 111, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/cache.py", line 38, in _cache_controlled
    response = viewfunc(request, *args, **kw)
  File "django/contrib/auth/decorators.py", line 21, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "wagtail/wagtailadmin/views/pages.py", line 150, in create
    validate_page_form(form, parent_page)
  File "wagtail/wagtailadmin/views/pages.py", line 311, in validate_page_form
    form.full_clean()
  File "django/forms/forms.py", line 353, in full_clean
    self._clean_fields()
  File "django/forms/forms.py", line 362, in _clean_fields
    value = field.widget.value_from_datadict(self.data, self.files, self.add_prefix(name))
  File "wagtail/wagtailcore/blocks/base.py", line 457, in value_from_datadict
    return self.block_def.value_from_datadict(data, files, name)
  File "wagtail/wagtailcore/blocks/stream_block.py", line 138, in value_from_datadict
    if data['%s-%d-deleted' % (prefix, i)]:
  File "django/utils/datastructures.py", line 319, in __getitem__
    raise MultiValueDictKeyError(repr(key))

Here is the body:

{
action-submit: Submit for moderation, 
author: The Late Show, 
body-count: 1, 
cover_image: 21679, 
csrfmiddlewaretoken: 5h4B0Z7TacaIIPPQDs1LyNuJlJp3o3Iq, 
expire_at: , 
go_live_at: , 
page_relations-INITIAL_FORMS: 0, 
page_relations-MAX_NUM_FORMS: 1000, 
page_relations-MIN_NUM_FORMS: 0, 
page_relations-TOTAL_FORMS: 0, 
search_description: , 
seo_title: , 
slug: dislike-or-not-dislike, 
summary: <p>The rumors are true, Facebook is creating a "Dislike" button.</p>, 
tags: Kriya,Gangiah,FrankieDuToit,TheLateShow,facebook,dislike, 
title: To "Dislike" or not to "Dislike"
}

It seems to be related to this:

http://stackoverflow.com/questions/5895588/django-multivaluedictkeyerror-error-how-do-i-deal-with-it

[gasman]

OK - I think this is caused by the editor using the ‘back’ button while editing. I’ve been able to reproduce it with the following steps (in Chrome):

• create a new page with a streamfield
• Add a block to the streamfield
• Navigate away from the page (e.g. by clicking ‘Images’ in the menu)
• Use the browser’s back button
• Submit the form

I don’t think it’s possible to eliminate this entirely - the browser is trying to restore the previous state of the form (including the hidden field that specifies how many blocks there are in the streamfield), but fails because the form HTML changes dynamically during editing. However, this will be mitigated by #475, which discourages users from casually browsing away from the page editor - therefore I’ll close this as a duplicate of #475.

[tomkins]

We’ve recently had this occur when sharing preview links, hopefully recreatable with #7392.