The inability to predict with any degree of certainly what those ever present clouds on the weather satellite pics are going to do. Packing my gear and travelling to an observing site takes an investment in time, energy and money. Many times clouds materialise out of nowhere once the temperature drops. Other times I have stayed home and the clouds have disappeared and I have missed an opportunity.
People who turn up to such sites with headlights blazing and then interior car lights on.
People who come over for a chat while using those overly bright head torches. While facing you and talking the blasted thing is shining straight into your eyes
Bright laptop screens! One fellow I know has got it right. He uses a clip on shield with external red led to illuminate his screen from the front and it doesnt bother anyone and works well.