Many times it's something very small or something omitted in a sequence that
can cause these accidents. Something as basic as a heading check for the
right runway can be lost in a last minute conversation between two pilots as
the airplane makes the turn from the taxiway to the takeoff position; a
sudden distraction; a last second joke; anything that breaks a normal habit
pattern.
Its SO easy!!!!!!!

It is... and my understanding is that's the reason for the sterile
cockpit rule. Incidentally, there was an off-duty pilot sitting in the
jumpseat in the cockpit along for the ride on the Comair flight.
Wouldn't surprise me if things got a little too relaxed that early in
the morning for just a brief moment.

--
Guy