Karan was meant to send a client an invoice and forgot. When the client called, he heard himself say, "The system never reminded me." Then something felt off. At the back of his mind was an argument: the Gita says that the doer is not me, that nature does everything. So where is my fault?
The argument is tempting, but it turns the Gita upside down. "I am not the doer" is there to lessen pride and dread. It is not there to let us slip away from a mistake. You have a share in any mistake, and seeing it clearly is how you move on.
What the Gita says about the doer
The Lord says that the modes of nature perform all actions, but the mind deluded by ego thinks, "I am the doer" (Gita 3.27). The knower of truth holds, even while seeing, hearing and walking, that he does nothing (Gita 5.8). That sentence is spoken by someone fully at work. The heat of the fruit and the chill of fear do not get hold of him.
विविधाश्च पृथक्चेष्टा दैवं चैवात्र पञ्चमम् ॥ १८.१४ ॥
The seat of action, the doer, the various kinds of instruments, the many different kinds of efforts, and here, destiny as the fifth.
But when the Lord lists the five causes of an action, he counts the doer among them: the base, the doer, the instruments, the efforts, and, fifth, destiny (Gita 18.14). So the doer has a share. He is not set at zero.
Why blame cannot be passed on
अज्ञानेनावृतं ज्ञानं तेन मुह्यन्ति जन्तवः ॥ ५.१५ ॥
The all-pervading Lord neither accepts anyone's sin nor virtue. Knowledge is covered by ignorance, and because of that ignorance, living beings become deluded.
The all-pervading Lord accepts no one's sin or merit (Gita 5.15). So one cannot say "nature did it" or "it was meant to be" and hand one's share to another. The share stays with you, and that is also why you can mend it.
What this is not
It is not a call to beat yourself up. Seeing a share and sinking into guilt are different. The Lord asks Arjuna to lift himself by his own mind (Gita 6.5), not to sink.
Nor does it say that the whole mistake is yours. There are five causes, and the other four are in it too. Naming them plainly is honest. Using them as a screen is not.
An example: Karan after the mistake
Take Karan again. He stops and says three sentences. First: "My share: I forgot to send the invoice, and I had set up no reminder." Second: "The other causes: the week was crowded, and the system's alerts were not working well." Third: "What I will do now: send the invoice today, apologise to the client, and make a checklist for sending bills."
There is no excuse in these sentences and no self-blame either. The mistake is recorded, the causes are counted, and the next step is settled.
Recall a small mistake for which you once blamed someone else or the circumstances. Write three sentences about it: what my share was, what the other causes were, and what I will do now. Then do the one thing you wrote in the third sentence.
Going deeper
To read the verses in order and to see the difference between an honest use of the idea and an alibi, read the detailed reading. The whole topic is in Who Is the Doer?.