Profiting From Experience

How could the Elders of Israel be in the very presence of God on Mt. Sinai, and "see God" as they are described in this week's portion to have done --- and still later have Aaron sink into repeated rebellions and his sons Nadav and Avihu treat God so profanely that God ended their lives over it? Shouldn't having "seen" God carried them forward with ironclad focus? Why is seeing NOT necessarily believing? And what can we learn from this section of Torah history to benefit from it?