In a 3E game, our Cleric jumped off a staircase to get behind an enemy... and found he'd dropped into a room with half a dozen hobgoblins in it. He was surrounded and down to about 6 hit points... but the hobgoblins only had 5.
We convinced him that his best option was to cast Sound Burst, centred on himself. And he did, and rolled 6 for damage - killing all the hobgoblins and leaving himself on 0.
In our 4E game, our Paladin is used to our Dragonborn using his Acid Breath and hitting a bunch of minions and the Paladin. It's generally considered a good trade, though it doesn't stop him complaining about "Stop spitting on me!" in general.
Our wizard has taken as many "Each enemy in area" spells as she can, but for the couple she has that affect allies as well, she's taken the Eladrin feat that lets her Fey Step a single ally out of the area she's affecting.
In a M&M game I ran, Piratecat and barsoomcore were playing characters who disliked each other. Piratecat was the scion of the God of Winter, whereas barsoomcore was descended from Fire Giants and hated the cold. Piratecat had a Selective Obscure power that let him blind the targets he chose within the area. And without fail, the targets he chose were "All the enemies... and the fire giant." The joke throughout the game was that barsoomcore's character still hadn't realised that Piratecat could choose who he affected, because he'd been blinded every single time. (One line I remember - "Now, we're in a bit of trouble, so tactically it would be a good idea for me to exclude the fire giant from the blindness this time. ... but I won't.")
-Hyp.