What were the pilots on Northwest Airlines Flight 188 doing that made them overshoot their destination by 150 miles? 

I wish I could say with certainty that I knew what was going on inside that cockpit.  But I don't.  However, since the cockpit is only so big, I know whatever they were doing they both had to be "in" on it.  You're not keeping any secrets at 37,000 feet sitting that close together.   So, whether they were arguing, sleeping, having sex, drugging or baking cookies, one thing is for sure the only thing they were suppose to be doing was flying the plane.  And it's clear, they weren't doing that.   Even the auto-pilot recording said "Hey stupid and stupider, we're flying over the airport...whatcha gonna do?!"  Apparently, they didn't hear that either.

As my momma would say "...Somebody is lying!"

Sidebar:  One other thing I still can't understand is why the passengers didn't know that the plane was an hour late; didn't they begin to wonder what was going on in the cockpit after being in the air an extra hour?