The original Halo had all kinds of "glitches" that pros would exploit to the point that it was just accepted as part of the game.
Instant stealth. Normally there's a delay when you pick up invisibility but if you fire a round you turn invisible instantly, so you would wanna fire a a single shot with the AR as you pick it up.
Overshield exploit. You're actually completely invulnerable while the overshield charges, you can use this fact to your advantage to exploit the map. One of my favorites was on Damnation, you could throw a grenade at your feet while you picked up the overshield and if you did it right and jumped on top of the explosion you could jump over a wall that led to the rocket launcher. Now you have a rocket launcher and overshield. Which brings me to my next glitch:
Grenade exploits! You could do SOOO much with grenades other than what they were supposed to be for. Basically you could use them to blow weapons and power ups to spots in the map where they normally wouldn't be. The one in Damnation with the overshield/rocket, the invisibility was on a walk above that room and you could knock it down into the rocket room with a well placed grenade. So to recap, a highly skilled player could get the overshield, launch themselves into the rocket room with a grenade, knock the invisibility down with another grenade, then they have invisibility, overshield, and rockets in under 5 seconds. I was on the wrong end of this once. Wasn't even mad.
Reload exploit. Simply melee when he reloads right as soon as he puts the clip in. You'll be able to fire a split second sooner than normal.
Double melee. This one is just straight dirty. This one I refuse to use against normal players, it's just wrong. Against tough competition, prepare for double AR butt to the face. All you do is throw a grenade immediately after a melee and then melee again. Basically you can get 2 melees in as fast as you can hit 3 buttons. Straight up dirty. This is for people you don't like, don't use this on your friends.
I'm sure I missed a few but those are the ones I can think of off the top of my head.