Answer:
Mutations are usually anomalies in the sequence of a particular strand of DNA. The change is the sequence can be of various kinds and their individual effects vary. Sometimes mutations occur but they are never expressed. In such cases, on the whole, the mutation has had no impact.
Changes in a single nucleotide can change the reading frame for protein coding regions and can cause misreading of the genetic code. For example, nonsense mutations caused by the exchange of a nucleotide leading to a stop codon can cause truncation of a protein. On the other hand, we can have silent mutations which show no effect because the nucleotide that has been exchanged does not cause change in amino acid sequence. So this would be an example of a mutation which shows no effect.