It was hard to muster a proper sense of indignation when you were confronting the same dignified witch who, twelve years and four months earlier, had given both of you two weeks' detention after catching you in the act of conceiving Tracey.

Given the fact that there is a Tracey, then that act of conception must have completed. So, either McGonagall caught them at exactly the right moment, or the Davises had just kept on going after they were caught...

No matter how it happened, this scene must have played out hilariously.