I can't believe some of the integrated guys I fly with who have spent €120k on their flight training but still use the spare headset left in the aircraft - that baffles me.
I teach guitar amongst other things and people who spend 3k on a guitar and then leave the same set of strings on for two years when a new set is under a tenner baffle me too. I won't physically touch some guitars to tune them up they are in such a disgusting state, I wrap my hand in a cloth. You have to see to believe what two years of crud and sweat does to a set of strings.