Coulomb's law says that as the distance between two electrons approaches zero, the force becomes infinite. However this is just an approximation, and electrons are better described as fuzzy clouds of charge distributed through space. But bringing two electrons close together would increase the replusive force between them. If the force is not infinite, what is it?
My guess is that as you force two electrons closer and closer together, at some point there is so much energy localised in one spot you spontaneously create a black hole. And presumably there would be some way to calculate this value?