I'll place in what looks like an obvious clue or item of importance that really isn't. I really like using Nystul's Magical Aura on things. They sometimes end up carrying the craziest things. A few years back I had one barbarian carrying around what he thought was a magical chair. He strapped it to his back, and the halfling liked to sit on it and ride.
My players hate swarms with a passion. Since they almost always play fighter-types, they have a tough time with them, and constantly whine about it whenever they may appear. The players also hate my tendency to bring back one of my player's former characters as enemies (they're always brooding, overpowered and morally ambivalent, plus they leave the party without dying, so...)