What kinds of traits are produced by most mutations?

Harmful traits are produced by most mutations.  Sometimes the mutation can create nonsense codons that do not produce any amino acid and signals the cell to end the process. So, this causes the cell to do different things than what it was meant to.  Most mutations are destructive and cells have a method of examining if they have mutated or not. Its only when the checking fails that the cell becomes mutated. 

Try and make the question more clear : )