Worms 2 (PC)

Roofing

Cavern terrains have an indestructable border on top, normally making it impossible for anything to go above the ceiling. Super Sheep, Homing Pigeon, Homing Missile, Bazooka and other weapons can go above a cavern ceiling, but Worms are meant to stay below. However, various ways have been discovered whereby a Worm can surreptiously sneak above the ceiling of a cavern map.

Methods of Roofing

Tunnel to the left or right top corner with Blow Torch (or clear the terrain with other weapons and make a Girder platform), then Backflip backwards and Parachute to the top. Requires wind not to be more than weakly opposing.
Make a ladder of Worms, piled so that each one is standing on the edge of the one below it. With the right positioning, this can be done with as few as just two Worms, and the one on top can simply Backflip up onto the roof.
Move a Worm to the edge of the map, with empty terrain above (clear if necessary using weapons; make a platform if necessary, using Girder). Stand on the edge of this worm with another Worm. Use Jet Pack to go straight up and over the ceiling.
Attach the Ninja Rope to the the edge of the map, with empty terrain above; inch the rope into a perfectly vertical state; then inch it slightly beyond vertical, and go around the top of the ceiling. This is can be quite difficult to do without practice.
Stand almost at the edge of the map, very close to the roof; lay a Mine at the very edge, then walk onto the mine and use it as a platform to Backflip up and onto the roof (before the Mine explodes).
Stand at the edge of a girder (or other vertical surface) on the very edge of the map, just below the roof. Do a bump-Backflip and quickly fire and explode a Sheep to blow you up and over the roof.

This portion of the video shows this glitch using the Super Sheep:

by CPGLitch (Video) and Worms Knowledge Base (Source)

Girdering an Occupied Location

Placing a girder in a occupied location (with terrain blocking) is possible. By placing a girder using this method will cause worms to fall through the terrain and in to water if done correctly.

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). This method can be used to girder-intersect an enemy worm, if you backflip underneath the worm to kick it up into the air and quickly click the girder in the space previously occupied by that worm before it falls back to the ground.

This is reportedly the only method which works for Worms 2, and girders can only be placed where your worm was standing. If you use this method while falling from a ninja rope, you can rope-knock enemy worms into the place selected for a girder prior to pressing spacebar, pushing them through the terrain. The same is also possible for your worm while on the rope, if you press enter. You can then rope through the landmass the girder will push you into.

Editor Notes: I have not been able to replicate this.

A demonstration on video can be found in the video below:

by Worms Knowledge Base (Source) and mdppr (Video)

Cluster Bomb Head Explosion

Debunked: This glitch may have been fixed in a patch, or it may have never worked. I have not been able to replicate it.

If you stand on an opponent worm’s head, aim directly downwards and tap the spacebar, the Cluster Bomb will sit on the worm’s head. When the timer finishes and the bomb detonates, all of the clusters will immediately explode, causing very large damage to the worm.

The same effect can be achieved by placing the Cluster Bomb under the worm when the worm is on a steep slope, but may not be considered a glitch due to the terrain physics.

by Worms Knowledge Base (Source)

Ninja Rope Through Walls

Due to a collision detection bug, the Ninja Rope only detects solid pixels so often. Due to this mechanic, the rope can sometimes pass through thin walls less than 8 pixels thick.

by Worms Knowledge Base (Source) and Devoluti0n (Source)

Indian Rope Trick

This glitch allows the player to fire the Ninja Rope in an angle not normally possible (typically, straight down).

To perform this glitch, you need to aim vertically down with another weapon, jump, and while in mid-air select the Ninja Rope (by pressing F8) and fire it. The game will not adjust your aiming angle while you’re in mid-air, thus you’ll fire at the angle you last aimed at.

If the player aims vertically down and performs this trick with a vertical jump (single Backspace tap), the Ninja Rope will be fired directly down. In this position, the player can safely expand or contract the Ninja Rope while maintaining a vertical angle.