Fertilization of an egg by two sperm
This phenomenon is called polyspermy and results in an embryo which is not viable. This is because it has more than the usual two sets of chromosomes (one from each parent), but one from the mother and multiple (depending on how much spermcells enter the egg) from the father.

To avoid this, most species (including mammals) have protective mechanisms against it. The so called cortical reaction which starts upon fertilization (the first sperm penetrating the zona pellicula) and releases cortical granules to prevent further sperm cells from entering.