Mistakes happen; not one of us is perfect. In some seasons, we may even make more mistakes than usual, but if you are a diligent and hard worker, the trend line bends towards less and less mistakes.
What if the trend line is going the wrong direction? What must be done if errors continue to increase in number rather than decrease? There should certainly be grace, especially for the novice, but eventually there must be improvement.
If you see only negative trends in a team member's performance, there should be warnings. Help offered if you're able. The line speaks for itself though, and it can only decrease so much before it affects the entire organization.