Suicidal Jackal
From Halopedia, the Halo wiki
To see a Suicidal Jackal, you must play the Halo 2 level Outskirts, there is a glitch where you can see a Jackal Sniper kill itself.
To see him, play through the level normally, but when you are past the alleyway with the Drones, proceed to the next alley, go through it a bit and stop, turn left and you should see a Jackal Sniper. Do not kill him. Go trigger a checkpoint. Go back to the Jackal. You should see him fall over and die by himself. It only works when you see him, and he sees you.
Also, randomly, you can see a Jackal Sniper kill himself on Delta Halo level. This Jackal Sniper will appear where the turrets are, right after the gondola. The Jackal will run, drift...and fall to the lake.