Why tell workers every time about their work-related mistakes

As a manufacturing shop floor supervisor, daily, I have to handle workers' mistakes. There were times when the non-compliance was minor, so I have decided not to tell them about it. I didn't want them to feel criticised. But there were times when I've told them. Their reply: why now do you scold me, and last time was not a big deal?

My way of handling it is not to punish. But to make them realise that I know whenever they do something wrong or right.

Making a mistake is not a big deal if you start taking steps in correcting it. The problem is when you do nothing about it, and in time you think that this is the new normal.