Before you read this I'd just like to say that this ran on about 10 thousand words longer then I thought it would be. It's worth the read, but if you don't I'll understand. It's also a fun little campaign to play.
The best double cross I ran was a fun, and they got even with the Johnson (in a non terminal manner).
A fixer calls them about a job from a Johnson. The Johnson wants them based on their street rep (And the fact that they have hit his compounds in the past, but they don't know that). They meet and he lavishly lays the praise on. The face and mage of the group both are a little worried about this "wonder Johnson" showing up outa nowhere with their names, but the two razorboys in the group decided to let their egos grow.
The Johnson tells them that the run is a simple data extraction form a government compound in Seattle. They've gotta break-in, grab a set of disks that have illegal information that might cost the mayor her job (it's records he needs destroyed, but the runners have a soft spot for the mayor after they rescued her and she bailed them outa a very tight spot later on. I should also note that the fixer has been bought out and is now working along with the Johnson). They agree on a really large sum of credits for the job (10K

up front, 90k

on completion). He also tells them that the Lonestar that are guarding the area are corrupt. He says they've been bought out and are planning to take the mayor into custody tomorrow morning, so it needs to go down tonight.
So, the runner being pressed on time decided that the two street sam's should head back to the doss and get their equipment, the mage heads off to try and get in contact with a rigger friend for support and the face stays back with the fixer to try and see what she can dig up about the Johnson. She hires a decker (through the corrupt fixer) and finds out that the Johnson's story sticks. The face is still not happy, but with 100K

in the air, the reputation of a friend in office, and the simple pleasure of eliminating corrupt cops (she's ex-Lonestar) she decides to check up after the runs done.
They all meet up a good mile away from the building and take stock. The sam's got all the armor, guns, and ammo they might need to take down the corrupt cops, the riggers there with the van for a fast escape, the mage is kicking back and relaxing in the van and the face is getting geared up for lone penetration on the building (No one else is needed, the distraction outside should pull enough attention away [<--their thinking, not mine])
They pull the van up to the building at around 2:00am and the face hops out to go infiltrate the building. She does amazingly well managing to get to the data in a manner of minutes rather then the hour she had planned out for sneaking and hiding. The group kicks back and decided to wait since she found the data so fast. Might as well let her get out before the fireworks start, right?
Well, the cops had gotten a tip that the shadow runners might be attempting to steal some data from them, and got a description of the van. A star agent on patrol sees the van and quickly runs back to the rest of the crew for support. The Razor boys see this and decide to run out and try and kill him quietly. A brief bloody scream latter the cop is dead; everyone on the block has heard that blood curdling scream. The face is just about out of the back of the building. The other cops run out and see the runners, 2 of the 5 (now 4) man team drop for cover behind some cars and open fire, the other 2 run back to the building to call for reinforcements.
At that point the face pops out and sees the two cops making a run for it. She drops down and kills one with a snap kick to the head, and as she turns on the other one she recognizes him. It's her ex-husband (their relationship is still strong, but her being a shadow runner made them have to break up, till she is able to pay off her debt and return to the world of the SIN'ers). He hasn't noticed that his partner has bit it, he's still running.
(Quick time flash) The face is has pretty much kidnapped her ex, the other 2 cops are dead, the 2 razorboys are fine, and the mage is watching the reinforcements pull up. The rigger goes to take off, but the van is hit with a rocket. The vans damaged and the riggers knocked out but still alive. They never do find who shot the rockets. The Lonestar quick response team has arrived, and they're packing mondo firepower. It's a five man team, one being a mage.
(Another quick flash ahead) The Mage is still in the van, but has taken some massive drain damage. She's knocked out, but she took down the enemy mage and one of the heavy enforcers who made it into the van. One of the razorboys has decided to save his own skin and beat feet after killing off two guys with almost as much cyber ware as him, and the other razor boy is doing his best to keep the face and her ex alive. That's when shit really hits the fan. 2 Knights Errant security response vehicles show up and open fire on the now very tired and semi-wounded runners. The face manages to get the Rigger awake and they do their best to take off. One vehicle gives chase, but after an amazing idea by the face, they ditch the van in an ally, great into the sewers, and go for it. The razorboys and the faces ex are now carrying the mage, the rigger is still groggy and rather pissed about leaving the van behind to be blown up (It was custom fitted with explosives and a napalm tank, thus when they got into the sewer, it detonated covering the manhole they went down in fire.) The run into a few rats and a few angry orcs, but luckily the face has some connections in the orc underground. Enough connections to get them out alive at least. They do lose some cash and weapons in the process, but it's better then dieing, right?
Oh yeah, that other razor boy who beat feat, what did he do? He managed to duck into a nearby building and was checking out the response team that came in. As he watches, the one remaining Lonestar responder is pretty much executed by the knight's errant officer and the media quickly shows up. He kicks on the Vid in the doss he's in and watches the report about what just happened.
"We're coming to you live from 5th and Main where shadowrunners have just attempted a break in of the mayor's personal office! The Lonestar assigned to this building are all dead, but luckily Knights errant was called in by the locals to deal with the problem. Do you have anything to say sir?" The video pans over to the officer standing there, and guess what? It's the Johnson! He drones on about how it was a good thing they were in the area since Lonestar is so incompetent blah blah blah...
A few weeks later after hiding out and getting in contact with a new fixer, they dive into the details of everything. He was a knight errant PR rep, they find where he lives and they also track down the fixer who's gone into hiding now that he knows they're not dead. They kill the fixer (no loss there) and then break into the PR reps house and kidnap his wife and children. They black mail him saying that the Razorboy recorded their meet with his cyber eye (he didn't), and that he got a recording of them killing the Lonestar officer (he did).
They managed to walk away with over 250K

from that run. Took 21 hours to play out. There were a few mini adventures that happened in the underground and again at their doss, but this has run far, far beyond what I originally thought I would say.
And what else is fun is that they still manage to extort the poor bastard once in a while. He's now a plot device who I use to try and get them killed quite often, but he's just too damn profitable for them to try and kill.
Edit: Took some spelling mistakes out and some grammar problems. I'll probably check it again when I'm more coherent.