Normally, I literally can't roll to save my life.
Last night, I was running one of the last few sessions of the Temple of Elemental Evil campaign. The PCs get ambushed by a sort of all-star jam of evil, consisting of all the bad guys who'd teleported out of the dungeon before getting slaughtered by the PCs. There's a mind flayer, a pair of fey, a barbed devil and a blackguard vampire. The blackguard vampire recently slaughtered half of a small group of soldiers loyal to the PCs, so he's got half-a-dozen vampire spawn with him.
The ambush takes place in a forest; the fey used Plant Growth spells to thicken the canopy so the vampires can move around.
During the fight, one player chucks a fireball and asks "any chance I can use it to burn up a bit of the canopy and let sunlight in?". I go "sure - say on a 15+ you burn away the leaves over this square and let light in. I'll roll for it."
The fireball must have been composed mainly of Agent Orange, 'cos it creates a neat window right above two-thirds of the spawn. Buffy-poof.