First person pronoun with a verb ending in 's' follows very definite rules. So to call it ungrammatical is nonsense, though it's not part of standard English. I'm from Australia and use it myself.

1. Informal context.
2. Relating a story involving the speaker.
3. Not all verbs are affected. 'Be' and 'Have' remain first person.

"I've just finished the shopping and I'm going to my car when I sees this man etc."