וימת נדב ואביהוא…בהקריבם אש זרה..ובנים לא היו להם (במדבר ג:ד)

Chazal say that Nadav and Avihu died because they didn’t have children.  In truth, their sin was “בהקריבם אש זרה” – they offered an alien fire.  But had they had children their punishment would have been pushed off for later. As it says “פוקד עון אבות על בנים (שמות לד:ז) – the father’s punishment is upon the children, and since ובנים לא היו להם  they didn’t have children, they were punished.