My apologies; that was a significant blunder on my part.

John: 'I'm very sorry, Alice. I missed the deadline completely.'<br>

Alice: 'My apologies; that was a significant blunder on my part. Let's discuss how to avoid this in the future.'

John: 'My apologies; that was a significant blunder on my part. I'll explain how this happened.'

Alice: 'The project budget was significantly exceeded, John.'<br>