My favorite anecdote I heard of from a flight instructor is that of a cargo plane taking off that had some heavy vehicle tied down in its hold. It broke free of its straps during takeoff and rolled to the back of the aircraft, which shifted the CG such that the plane entered an unrecoverable stall and crashed.
There is a video which makes its way around the Internet regularly of this crash[1] which was attributed to exactly that, killing all seven on board. (Which makes it pretty heavy for an anecdote, admittedly.)
