What causes Internal server error on accessing some questions?

edit

After upgrading to 0.7.50 we get and internal server error on loading some questions:

Upgrade steps:

pip install --upgrade askbot
python manage.py migrate --list    
python manage.py migrate
python manage.py syncdb --migrate
python manage.py collectstatic

plus:

python manage.py generate_post_snippets

The error

    list index out of range
    Request Method: GET
Django Version: 1.5.12
Python Version: 2.7.6
Installed Applications:
('longerusername',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.sites',
 'django.contrib.staticfiles',
 'django.contrib.admin',
 'django.contrib.humanize',
 'django.contrib.sitemaps',
 'askbot',
 'askbot.deps.django_authopenid',
 'south',
 'askbot.deps.livesettings',
 'keyedcache',
 'robots',
 'django_countries',
 'djcelery',
 'djkombu',
 'followit',
 'tinymce',
 'group_messaging',
 'compressor',
 'django.contrib.messages')
Installed Middleware:
('django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'askbot.middleware.anon_user.ConnectToSessionMessagesMiddleware',
 'askbot.middleware.forum_mode.ForumModeMiddleware',
 'askbot.middleware.cancel.CancelActionMiddleware',
 'django.middleware.transaction.TransactionMiddleware',
 'askbot.middleware.view_log.ViewLogMiddleware',
 'askbot.middleware.spaceless.SpacelessMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware')
Traceback:
File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py" in get_response
  113.                         response = callback(request, *callback_args, **callback_kwargs)
File "/usr/local/lib/python2.7/dist-packages/django/utils/decorators.py" in _wrapped_view
  91.                     response = view_func(request, *args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/askbot/views/readers.py" in question
  513.                                 user=request.user
File "/usr/local/lib/python2.7/dist-packages/askbot/models/question.py" in get_post_data_for_question_view
  1087.         post_data = self.get_cached_post_data(user=user, sort_method=sort_method)
File "/usr/local/lib/python2.7/dist-packages/askbot/models/question.py" in get_cached_post_data
  1195.             post_data = self.get_post_data(sort_method)
File "/usr/local/lib/python2.7/dist-packages/askbot/models/question.py" in get_post_data
  1245.             first_rev = post.get_earliest_revision()
File "/usr/local/lib/python2.7/dist-packages/askbot/models/post.py" in get_earliest_revision
  1506.         rev = self.revisions.order_by('revision')[0]
File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py" in __getitem__
  231.             return list(qs)[0]

Exception Type: IndexError at /question/198/from-windows-xp-to-windows-7/
Exception Value: list index out of range