Here is a tragic incident, from my home town, in which a man was killed by one punch. There were two incidents separated by several minutes. In a pub, the first incident saw man 'A' strike man 'B' on the chest. Man 'A' was ejected from the pub. A typical incident we see day in day out.
Man 'B' dwells on the incident and some several minutes later leaves the pub and finds man 'A'. He punches man 'A' who falls to the ground. Man 'B' immediately turns himself over to the police. Man 'A' dies the next day due to bleeding on the brain.
The link contains the details of the case.