Code igniter transaction don't rollback on SQL error

But standing to the documentation, if i use $this->db->trans_start() & $this->db->trans_complete(), the queryies will rollback automatically when one of those fail.

Why my records are affected the same when one query fails?

I've noticed the engine of the table affected by the transaction was MyIsam, and than i changed it to InnoDB, ( because MyIsam not support transactions), but the results are the same. The queryies never rollback on a failure state.