Pac Swaps (Or, How I Learned To Stop Worrying And Break The Game)
1. The Prep
First things first, you're going to want to pick up some armor. Head over to any clothing store with a tops section, scroll down to "Utility Vests" near the bottom of the list, and buy either color of the Heavy Utility Vest. This vest will increase your resistance to damage when in heists, at the expense of your ability to sprint. Fortunately, most heists are vehicle-heavy, so sprinting isn't really necessary.
Now, to activate this bonus, you'll need to be able to equip the vest. To manage this, you'll need to select Player Saved Outfits as the outfit selection when starting the heist. This is a customary thing for more than just this glitch, and there are many people out there who will be (perhaps rightly) very upset if you don't set this. While you're at it, change the difficulty to Hard; it won't be that much more difficult, and you'll make an extra 25% of the final payout.
What payout? Well, I've hinted at it, but I haven't actually stated it. This glitch allows you to replay the Pacific Standard Job finale as many times as you want without needing to complete the setups. In order to make this work, you'll need two players to have the heist finale ready, and two more as fillers. The two with the heist are referred to as "cohosts," as each run the host will not be able to receive his cut. To circumvent this, the current host takes a 0% cut, gives a 50% cut to the cohost, and divides the rest into two 25% cuts for the fillers. The cohost will be the host of the next run, repeat ad infinitum.
The heist itself is easy. When you enter the bank, one crowd control player will hold an RPG in the lobby. This keeps the intimidation meter at its maximum; you don't need to shoot or anything else. The other crowd control deals with the cops behind the counter and upstairs; the hacker and demolition proceed through the checkpoints as normal. It's important that the hacker take all of the cash; this will decrease the amount of targets that can be hit during the escape. Once all of the cash is taken, meet back in the front entrance for the cutscene.
After the cutscene, the hacker should take the money further into the bank to avoid getting shot. The two crowd controllers and the demo should exit and deal with the cops outside. Then, one of the CCs will have to run down the alley route, avoiding the cops as much as possible and heading towards the bikes. Once he gets there, he must blow them all up, then commit suicide. When the party reaches the menu, quick restart; this will respawn everyone in the alley with the bikes.
From here, the party will split. Three players (one of which is the money holder) follow the map on the left of the following screenshot, and loop back into the alley where one of the setup missions took place. In there is a set of ladders that lead to the nearby roof; this will make a safe waiting place while the fourth player does his job. That fourth player will be going to Vespucci PD, a short ways southwest. He'll approach along the red line in the lower-right map, shoot a cop in front of the bank, follow the green line away, then head back along the red to the PD again. Once there he'll climb onto the black section of railing shown in the upper right of the below image, then jump down onth the balcony in front. From there he'll climb up a series of ladders, reach the roof, and steal a Maverick helicopter from on top, then return and pick up the group. From there, it's a smooth flight to the boat.
3. The Glitch
Once you're landed and all in the boat, the host is going to need to perform some trickery. You see, in order to make this work, he has to disconnect at just the right time, allowing everyone else to get paid but not losing him his finale. To do this, he'll have to force his game to close using Task Manager or a batch file, or disconnect his internet until the game returns him to singleplayer. The time to do this is just after the bridge, no more than a second after the HUD fades. If done right, once he loads back into the game he'll receive a message from Lester about the finale, and the whole shebang can start again.
The advantage to this method is, due to the way Rockstar handles heists, there's no way for them to patch this out without seriously reworking their network code. There's also no way for them to justify wiping the money, as it could just as easily be a network error. That means that, short of flat-out telling Rockstar that you're exploiting an error in the net code, there's no way of actually getting caught.