Most overused in: Operation Flashpoint: Dragon Rising, Borderlands, Red Faction: Guerrilla, Killzone 2.
This is an easy one: no matter where you go, no matter what war you fight in, there'll always be barrels. Barrels apparently filled to the brim with highly unstable explosives that react to just having the word 'bullet' whispered at them to send a mushroom cloud of death skywards and enemy flesh-chunks bouncing across the floor.
The only thing more certain is the fact that guards never realise the danger. Instead, their first (and last) thought when you burst out shooting will always be, "I'll be fine behind this 60 gallon drum of aviation fuel."