title: Django admin: missing inlines for some records
body: <p>I'm having some problems here with django admin site and inlines. I've been googling for solution for two days now but nothing.</p>
  
  <p>I have two models:</p>
  
  <pre><code>class Measurement(models.Model):
    user = models.ForeignKey(User)
    date = models.DateTimeField(auto_now_add=True)
    # etc
  
  class Media(models.Model):
    measurement = models.ForeignKey(Measurement)
    link = models.CharField(max_length=255, blank=True)
    description = models.TextField(blank=True)
    # etc
  </code></pre>
  
  <p>And configuration for admin site:</p>
  
  <pre><code>class MediaInline(admin.StackedInline):
    model = Media
    extra = 0
  
  class MeasurementAdmin(admin.ModelAdmin):
    inlines = [MediaInline,]
  
  admin.site.register(Media)
  admin.site.register(Measurement, MeasurementAdmin)
  </code></pre>
  
  <p>The strange thing is: admin site shows inlines for some Measurement objects and doesn't for some others. Whole formset is invisible (it's not there) even if related records are visible in database. Also, <code>ValidationError: [u'ManagementForm data is missing or has been tampered with']</code> exception is raised for these objects. Have anyone dealed with this?</p>