Septic tanks
I once had a Charter to pick up a large Yank heading a drilling team who was about to be lynched by the locals. When I arrived he announced there was no way he was gettin on board as "there was no F/O and the contract requires two crew....or don't you know that", I responded that this aircraft was leaving in two minutes..with or without him. You guessed it he got onboard and proceeded to piss and moan about anything and everything. Unbeknowns to him I had slipped an Oxygen tube into the corner of my mouth and steadily climbed to 16000 Ft. Slept like a baby and stayed that way till POM.
Thank goodness for un-pressurised aircraft.