(Not a reply, but further information on my confusion.) In today's passé composé vs. imparfait lesson, the phrase cited in my question is translated as "Nous n'avons jamais oublié cette leçon." If one has never forgotten something, doesn't it indicate that it is ongoing? It's not a one-time action, or even something that has passed. Is it because it was an event (the snooping and being caught) that "taught" the lesson and since the event is in the past it's passé composé? 

Well, if you are talking about the act of forgetting it is an instant in time: there one moment, forgotten the next. It is the *state* of not remembering anymore that is ongoing. For a French speaker, it is the act of forgetting which hasn't happened. It's negative and therefore a bit more difficult to wrap your head around but it does make sense on some level....if you start thinking like the French. ;)

Thank you. Explaining that "...the act of forgetting is an instant in time..." and that even though it has NOT happened, forgetting is still seen as something that happens once was very helpful. How I wish I could think like the French! (And be able to explain it to my students!)