Once you begin the level, jump on the hill with the Grunt on the Plasma Cannon. Jump up until you are on top of the map, and then go to the bridge area. Walk into the void where the bridge would be, and you will find that you are not falling.
There is also an invisible bridge glitch on the level Regret. First, progress as normal to the building where you fight Regret. Before the cutscene commences, get the gondola near the platform of the building. Once there, hurry and jump over the area the bridge is supposed to set up. If done right, you will activate the cutscene with John-117 walking on thin air. You will also be able to see the Marines, if they were at the boarding platform. They will not follow you or assist you, and will remain on the Gondola. Additionally, the Gondola will not continue towards its docking station, nor will the station extend out to accept the Gondola.