In a perfect world... no one would make mistakes.
It was a combination of things here... The electrician forgot to replace the cover... for whatever reason. The old lady was partially sighted... if she had better vision, or someone caring for her doing everyday things like having a shower.. she would have seen the cover missing, reported it, and that would have been a near miss.
There is little point now slating or defending the spark in question.... There are policy's in place that should stop this from happening... but no policy is perfect.
Luckily, there was not a fatality, but things could have been so much different.
Maybe new policies will be put in place... such as 2 man teams, or extra tests developed to ensure everything is double checked before leaving a property.... Such as a minute walk round before leaving, as suggested.
I wouldn't like the excuse "everybody can make mistakes" if i'm on a bed awaiting brain surgery!