Even in the most stable conditions cells undergo mutations. So in humans(an example) with millions of cells, mutations must be a common affair. But how is that we are still basically the same throughout the life, and no random mutations cause sudden disorders in us. Even if mutations are rare, they cannot be nil and how are they controlled.
There are many reasons why humans rarely experience the deleterious effects of mutations. Many mutations arise from errors in normal biological processes such as DNA replication or recombination. DNA repair mechanisms and apoptosis (programmed cell death) are some ways of 'dealing with' mutated or damaged DNA. Another reason that we rarely see deleterious effects of mutations is because of the way amino acids are encoded.
There are approx. 20 amino acids encoded by the genome, yet each amino acid can be encoded by approx. 4 different codons. For example threonine is encoded by ACU, ACC, ACA, ACG. This means a point mutation is unlikely to make any difference to the final protein product as there is a high chance that the mutation will still code for the same amino acid. This is what is called a silent mutation, and there are other types mutations that do not have deleterious effects.