Alien Fire[1]
ויקחו בני-אהרן נדב ואביהוא איש מחתתו ויתנו בהן אש וישימו עליה קטרת ויקרבו לפני יקוק אש זרה אשר לא צוה אתם: ותצא אש מלפני יקוק ותאכל אותם וימתו לפני יקוק
The sons of Aharon, Nadav and Avihu, each took their fire pan, put fire in it, placed on it incense, and offered before Hashem an alien fire that they were not commanded [to bring]. A fire then went forth from before Hashem and consumed them and they died before Hashem[2]
Right after the inauguration of the Mishkan, the Tabernacle, tragedy strikes. The entire Jewish people are overjoyed that their hard work has paid off. They’ve finally built the Mishkan, and Hashem has shown that His Divine Presence is with them[3]. Two of the sons of Aharon, wanting to express their gratitude, offered a voluntary incense offering. Their plan backfires and a fire comes forth and kills them. The verses seem to indicate that their sin was bringing an unwanted offering. However, Chazal indicate that there were other sins which caused their deaths.