I once was in charge of Technical maintenance in a large manufacturing plant that carried out P.M. during shut down, unfortunately it was carried out by the maintenance electricians and in spite of attempting to 'educate' them, they would do things like replace memory back up batteries, but did not leave the controller powered, thus resulting in the parameters lost and would have to be reloaded, usually discovered when production resumed!
One instance the guy on the night shift required batteries for his flash light and took them from a machine back up set.
Just a few of many horror stories.
Max.