Is it possible for a change of a single nucleotide in DNA to have an observable effect on phenotype?

No change occurs in phenotype.

Some mutations don't have any noticeable effect on the phenotype of an organism.

new evidence is discovered.

oversights and errors are corrected,

new technology is applied to evidence.

martin luther believed that selling indulgences was wrong and gave people a license to sin. the roman catholic church used the practice of indulgences to raise money for major building projects.