Girdering an occupied location
From Worms Knowledge Base
The Girder placement glitch is when a girder is placed anywhere, with complete disregard to terrain, worms, and objects that would normally be in the way. Any worm or object that the girder is placed on will fall through the map until it reaches an open area beneath it or falls into the water.
How To Perform The Glitch
To perform the glitch, select a target using Homing Missile or Homing Pigeon (but do not fire). Then, select the weapon before girder. Then you need to precisely hit F7 and Space at the same time. (If you are on a Jet Pack, Ninja Rope, Bungee, or Parachute, hit F7 and Enter instead of F7 and Space.)
Another method to do it is to choose the weapon before the girder, then jump. While in the air, press F7 and click where you want before you land. Once you land, press spacebar to place it. (prior to pressing spacebar to place it, you can walk and jump around the map like normal)
Affected games
This glitch is present in Worms World Party, and has been fixed in the 3.6.19.7 W:A Beta update (after a partially failed attempt at fixing it in 3.5 Beta 1).