Will a well-trained worker sometimes perform unsafe acts?

Of course they will!

People often come to work with a sick child at home, or after a fight with their spouse, or worried about how to pay the mortgage, or with a thousand other things haunting their attention.
Sometimes they'll take a chance when it's close to break time or if they don't want to take the long walk to the tool shed.
If their glasses fall onto the ground or into a tank, they'll forget that they are in the path of a machine.

This is why we need guards around pulleys, lock-out/tag-out rules, multiple & redundant levels of safety, rules against never working alone, etc.