A population will never exist in Hardy - Weinberg equilibrium if
According to Hardy - Weinberg’s principle, the allele frequency remains constant from one generation to the next if the population is large and randomly mating. In case of selective mating, certain alleles which are recessive may become further rare in the population thus changing the allele frequency.
Mutation can bring a change in the nucleotide sequence due to which there is a possibility for the production of new proteins or no proteins at all. This ultimately results in a change of phenotypic characters.
Gene-flow can happen out of the population if there is migration of gene pool from one place to another.
If the population is large, any change in the allelic frequency of an organism due to chance or accident is insignificant.