On the bottom of Midship, there are two energy shield type things that glow bright turquoise. They look like 1/4 ovals. Have one person stand facing the shield, touching it. Have another person assassinate the first person. The first person's body will fall partly into the shield. Whichever part falls in, becomes uncontrollably shaky and deformed. Sometimes, heads will stretch out until they start flipping around like balloons on rubber strings.
The little cylindrical structure, medical station on the bottom level of Turf is one of the glitchiest places in Halo 2. Have one person stand inside the structure, and another person assassinate the first person while staying inside the medical station. If you do this right, the dead body will flop around and slap himself in the face with his arm. To make the person slap their self, they have to be lying on their side. If they are on their back, the arms will only constantly twitch.
Well, as fans of the Webcam can appreciate, the power went out in our building about an hour ago, so I am typing this from the basement of a Kirkland abattoir. And that, ladies and gentlemen is why it's short and lacking any real art. I lost an entire day's work. Sorry. I don't have access to our servers. So the Bungie guys and gals took the afternoon off and headed to the local Tiki bar.
First, it is possible to hop back thru the invisible wall. Just Ghandi-hop ( crouch-jump + crouch again) onto the small ledge on the wall of the level, and jump right past the concrete wall back on to the roof.
Second, it is possible to get past the invisible wall without a "dummy-guest". Just have your friend who already got thru stand in the same corner on his side and jump. The guy who is still on the roof side sword-cancels him and squeezes thru the crack no dummies required.