Recently, on a Cargo 747 Classic, the Cargo was not tied down correctly.
A giant drill bit used to DRILL FOR OIL slid OUT off the pallet and PUNCHED through the pressure bulkhead all the way through to the rear stabilzer!
The airplane was able to turn and return to the airport. It's now stuck in Africa in the middle of nowere with a Halliburton drill bit stuck between the fuselage and the Horiz. stab. Lesson learned: make sure your stuff is tied down and the loadmaster looks over all cargo to make sure the unlikely cant happen.

Whats the craziest, scariest or luckiest thing that ever happened to you flying?