If MySQL has a default encoding of UTF8 the unique_together index key is too long for the Scribble model:
class Scribble(models.Model):
...
slug = models.SlugField(max_length=255, blank=True, default="")
url = models.CharField(max_length=255, blank=True, default="")
content = models.TextField(blank=True, default="")
...
class Meta(object):
unique_together = ('slug', 'url')
Neither field is more than 255 characters, but with UTF8 encoding MySQL reserves 3 bytes for each character. And as the unique_together is set for two fields, that's 255 x 2 x 3 = 1530.