Parshas Achrei Mos begins with a description of the Sacrificial Service of Yom Kippur. The first thing Moshe was told to tell Aharon is a negative: that if Aharon were to come into the Kodesh Kadashim outside of a specific time and without specific korbanos it might cause his death.
Reb Moshe, in his second English Darash, says that it seems that the first thing in the instructions for Avodas Yom HaKippurim should be a positive- "Do X Y and Z." It is unusual that the first statement in the instructions is "If you don't do it correctly, walking into the Kodesh Kadashim can be deadly."
Rav Bergman ties this idea to the death of Nadav and Avihu. He says that what ultimately destroyed them was the fact that after repeated contact with Moshe and Aharon, and even with Gilui Shechina, they lost a degree of awe, of hispaalus, and this diminished hispaalus led to their fatal errors (Shemos 24:11, ויחזו את האלקים ויאכלו וישתו, and the Gemara in Sanhedrin that immediately precedes the one we just quoted about familiarity: אמר לו נדב לאביהוא אימתי ימותו שני זקנים הללו ואני ואתה ננהיג את הדור).