Sometimes you are less than careful and a small change leads to being stuck in a not so nice smelling pool of biologicals.
Well some people have not so good days, some bad days and some worst.
The customer have end point security and it checks the internal clients if they are valid and updated anti-virus.
I has doing to OS upgrade for the anti-virus server and the clients were not updated, so they slowly got expired and the end point secuirty lock them down. This could be have been easily explained if not for the the Primary DNS works which caused the end point security to another lock down when the Primary DNS did not response.
A most less than desired sequence of events.
Well I hope this does not turn out as a dear old lady put "anus horribilis".
To restore customer confidence, stick to after hours work and perform more end to end checks.