Classical Sanskrit was defined by the grammarian Pāṇini and exemplified by the language of the epic literature especially including the Mahabharata and the Rāmāyaṇa. But the theory was always tighter than the practice, and the language of the epics includes non-Paninian forms, usually influences from spoken language(s), which eventually became Hindi/Urdu and other north Indian languages.
But it’s a little bit of a problem: how do you handle these exceptions? Traditional scholarship calls these forms “ārṣa”, “of the ṛṣi”. As I understand it, it’s a case of “do as I say, not as I say.”