In the last episode (S04E21) of The Vampire Diaries, Silas himself (not his mental projection) get inside Caroline's house and injured her mother. Caroline's mother owns her house and Silas was never invited in. Then, how did he get inside? Is there a loophole to this rule of invitation?
As he is the very first immortal and a warlock, the rules might not apply to him. He might not, despite needing blood, be the same kind of vampire that the Originals are. As all other vampires in the world descend from the Originals, the rule of invitation might only apply to them. They were, after all, created by different witches. Alternatively, Caroline's mother might have invited him earlier believing he was someone she knew, allowing him to enter easily this time.