Walter watches Jesse’s girlfriend Jane choke on her own puke as she’s overdosing and does nothing to help her.
He was sneaking around in their home and did not want to risk getting caught, so he almost intervened, but then decides against it, and regrets doing nothing as she dies in front of him.
He later reveals this to Jesse during a conflict.
From Jesse’s perspective, he had just woken up and she was dead, choked on her own vomit.
Walter tells him he watched it happen and essentially admits that he could have helped.