While mRNA stands are being created a sequence is sometimes miscopied. What is the best possible outcome, the worst possible outcome and the long-term effect on the cell if mRNA strands were miscopied?

The worst possible thing that could happen to a cell and the long-term effect on the cell is that the cell will not make functional proteins from that mRNA strand. The best possible outcome for the cell should take place is that the miscopied sequences codes for the same amino acid as the original sequences.