Last question first: As a development chemist doing lab. work to scale-up his own procedure in the pilot plant, after 15 years of this I would say always expect the unexpected. Your example says it all. Do the bags contain exactly 50lbs, no they don't, how do the operators measure half a bag? They can't. So I hope that process is robust. It is certainly not conforming to cGMP. Furthermore lofting 50 lb bags around cannot be good for their backs so they will eventually be off work with "back pain". Our balances had printers so the weights were actually recorded as part of the batch record.
I've seen valves being left open and solvent being pumped in at the top, you get a phone call "Dr Disco the reactor is not filling up, what do we do?".
Valves being fitted on the wrong way round on cylinders, i.e. the output valve was where the gas inlet valve was. This was discovered halfway through adding DIBAL-H. So we had to stop, call a mechanic get him to make a new valve fit it and somehow carry on, all the time the reaction is stirring away in the background.
Just a couple of examples.
To get your operators to alter their methods of addition or their practices in general you are going to have to change the philosophy of the plant which will not be easy and will meet with great resistance. But the operators, although usually a great bunch of people, will always find a way round this to make life easier for them.
Basically it means that you are going to have to stand there and watch over them and jump when you see something wrong, because if they can take a shortcut they will do it.
In Pharma our operators were trained in cGMP so I had the feeling they would not take a dump without a written procedure and then documenting it. However they still took shortcuts when they could, unless you were there at every critical phase of the process. Not with safety though, there they were good, but with things like running the temperature up too fast, or cooling too slow or fast. This got better when the equipment was computerised, but they still made "mistakes".
Get to know the operators, chat to them. Get the plant manager to give you a tour of the plant, ask intelligent questions, Show an interest in their problems, just generally be human not a boss. I found that this worked quite well, especially when I compared my approach to that of some of my colleagues, who never showed up to see what was going on, who, when they did acted like little Hitlers stomping their feet and giving orders.
Anyway I've ranted on enough, just always expect the unexpected.