Choosing to wait during your turn
Penalties in darkness
Attacking with a non-intended weapon - chairs, torches, bottles, etc.
Hitting a non-intended target
Fighting while mounted on a beast
Actions and penalties a character can take within their turn
Use of racial features in combat: Claws, Fangs, Horns, etc
Attempts to render a target unconscious instead of killing
Using weapons or punching with non-dominant hand
Bonuses and penalties for characters in prone
Physically shoving a target
Firing ranged while engaged in melee combat
Drawing, holstering, etc is a free action
Penalties when firing multiple shots
Reloading ammunition into a firearm
Mechanics for shotgun spread
Mechanics for attempting to hit creatures or characters both large and small
Hitting fast moving targets
Stunned character penalties
Making things difficult for foes through acts of humiliation and intimidation.
Touching a target instead of outright attacking
Duel wielding weapons
Unarmed defense against melee weapons
Attempting combat from an unstable platform: Mount, train, vehicle, etc
Fighting on or in vehicles
Recklessly attacking
Attempting to move out of melee combat - (opportunity attacks)