>>295869 (OP)
Autistically spending an elaborate amount of time min-maxing my non-unique gear and subtle upgrades / making shrewd trading deals to get the maximum amount of money, before realizing I'm playing a game which auto-inflates difficulty and economy based on your level and amount of money. Mass Effect, Knights of the Old Republic, and Crusader Kings is notorious for this, out of many.
Reading every book and doing every conversation tree (even ones I don't care about or am doing in a New Game Plus / 2nd playthrough) in case there's a hidden achievement, perk, permanent bonus, or dialogue option which your character remembers for you 20 hours later.
Carefully-placed headshots, yet the game's ballistics aren't realistic so the bullets cartoonishly drop or veer off (unmodded STALKER and Fallout 3+).