one form is a hardware glitch. if the host hits the standby button on their modem, it will lag everyone out to the blue screen, but the host can continue to run around. they can walk around for probably 30 seconds with no one else in the map. then once they get the flag, they hit standby again and everyone returns, but the flag has mysteriously been already taken to score.
the dummy is more of a glitch, but it is considered cheating. basically a person is invisible to everyone else. to everyone else, he will be standing still wherever a warthog picks him up, but on his screen the warthog has moved him to wherever he wants. however if the "dummy" moves, he will teleport back to where his actual body is by the original place the warthog picked him up. but you can do this for example.
-dummy glitch outside one base on coagulation (the player model will be by his base, but the invisible/dummy guy will be in the warthog).
-they drop him off right outside of the other base.
-someone grabs the flag and throws it to the dummy guy, or drops it right next to him.
-the dummy holds x and picks up the flag
-the dummy then moves which transports him back to his model, right next to his base.
-he brings it 10 feet into his base and scores.
however if the other team finds the actual model and kills it, the dummy will die.
these cheats aren't ones that come up accidently, these people are looking to find glitches in the game and take advantage of em.