Acid Arrow Launcher (4e Trap)
From D&D Wiki
A magical object taking up one square, such as a pillar with a green orb mounted on it, that fires a shimmering arrow of green, glowing liquid that streaks to it's target and bursts in a spray of sizzling acid that strikes any creatures next to the target. Once the pillar is destroyed the orb will fall and crack, harmlessly shattering into worthless glass. The Trap attacks any that come within it's range (20 squares) unless it has been enchanted to not attack them.
Players can make an Knowlage Arcana Check, DC 15 to identify it when they see it.
This could be used in an encounter by positioning it so that players are within range while fighting monsters, making the encounter more difficult as they must try to avoid the Acid Arrow attacks. This would work particually well in combination with monsters that are immune to acid damage, as it can attack while they are adjacent to the target, as they will be immune to the secondary attack.