Early Banshee

It is possible to fly an Early Banshee in the level Assault on the Control Room.

Walkthrough
First of all, make sure you have both the Sniper Rifle and the Rocket Launcher. Then, when you reach the area where the Scorpion Tank is, finish off all of the Covenant forces. After clearing the area of all hostiles, leave the Scorpion alone or get out of it if you used it in the clearing effort.

Next, walk down the small hill and look up while doing so. You should then notice a small rock bridge followed by a small platform coming out from the canyon wall. Keep walking slowly until you get a perfectly clear view of the platform. Take out your Sniper Rifle, zoom in all the way, and try to aim at the middle part of the platform. Then, without moving, switch to your Rocket Launcher and fire. The end result (if your aim was perfect) is the Banshee that would have been used against you by a Covenant Elite should tumble to the canyon floor. This may take a few tries and some re-aiming, but this can work. You can actually use the Banshee, so have a blast and take advantage of a time saving hint later on. This trick can really come in handy in a co-op Campaign. Imagine it: your buddy and four Marines in the Scorpion and you in the Banshee providing air support (or vice-versa). Later, when you exit the tunnel, there will be a large tower. Fly to the top and there will be another Banshee sitting on top waiting for you to steal it before the Elite spawns. And in the second canyon, if you fly there before the enemy forces spawn, there will be a third early Banshee where you can take it or flip it over so it can't be used.

This trick can also be a great headache saver. When you get to the second canyon, fly the Banshee over the towering rock building in the middle. Then just keep flying very high until you get to those twin bridges. Land on the second bridge (the one further from you on approach). Simply go through the doors, but make sure the arrows are pointing the right way. You can finish the level with no more Covenant forces to deal with.

Another way to do this is near the end of the level, at the beginning of the chapter 'If I Had A Super Weapon...'. There are two Banshees sitting on the bridge that you walk out to. It is possible to obtain one either by killing the pilot (the Elite which tends to run towards the Banshee),simply by rushing up and getting in, or if you have a rocket launcher, you can blow the Banshee off the bridge(this can flip it over too), and you use the spire to get to the ground. This saves you from having to kill all the Covenant surrounding the control room, and you can have additional firepower on your way to the room itself.