First, take a look at this submitted preparation. The whole cavosurface margins are very wobbly. Now this seems that the operator was trying to smooth the margins but was clearly unsuccessful at it.

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ERRORS

1. Unstable handpiece. Ineffective hand rests.
2. The operator tried to smooth the margins but was unsure of how to do it well.
3. When refining the walls, the operator only focused on one point that has irregularities that still leaves the area with more irregularities.

TROUBLESHOOTING

1. This can be fixed without fear of enlarging the cavity preparation. You may use a hand instrument to create a smooth, flowing margins.
2. You may use a bur. But make sure to maintain firm finger rests and bur angulation. Practice first with a non-moving bur with wider strokes (about 3mm). The idea here is to smoothen the irregularities by moving the bur in wider, longer strokes.

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3. Remove only what needs to be removed. For example whenever there is a sharp corner, only smooth that out.