Was the chip real or fake?

1. Why was the fake chips stolen by the Horsemen? Because the real chip had been stolen prior.

The Li from the Magic shop was the guard walking behind the Four Horsemen just before they are passing by the security metal detector in the science building. So presumably Li had stolen the real chip before them, as he is part of the EYE community...along with the other guys.

2. How does the fake chip become the real chip in the airplane?

When both Li and his grandma learn about the Horsemen's plan of revenge in the magician shop they probably can predict their plan already. Grandma starts to talk in English and puts the chip back to the Joker card (stolen by Li prior) by playing with the card. That is when the real chip is back in the Joker card. Then Walter checks it on the airplane and finds that it is real. As he co-designed and invented the chip, it is hard to believe that one can fool him with a fake checking device.

Also, if Walter discovers it is not the real chip on the airplane, I am sure he would believe that one of the Horsemen will still has the chip and therefore will NOT throw them out of the airplane but rather kill them or torture them.

The Horsemen stole a fake in the first place.

The "host" guy works for the Eye (the one who asks where he has seen Atlas before), as shown at the end of the film. He has already procured the chip by the time the Horsemen show up, who steal the fake chip he has placed. Notice that when the host orders the techs to run a diagnostic while the horsemen are searched, nothing happens (despite the chip having been replaced by a hair comb). The host must have sabotaged the system already, because it is incapable of recognizing whether the chip is fake or not.

The Eye personnel simply needed to swap out the fake with the real one before the Horsemen boarded the jet.

I agree partly with so many of the answers, but people are overlooking a major part. THE EYE was organizing so much of what was being done. At the end of the movie, there are plans for just about everything that they did, including the OCTA event where they got kidnapped in the first place. Which means that they knew what Walter was planning all along. Thaddeus tells Dylan that the OCTA event was setup to expose Walter, because they knew he was in hiding. The disc is fake when they steal it, because they need to make sure that Walter doesn't get the real one, but needs to think that he did. But THE EYE also wants to see if the Horsemen can work organically, which is why the head attendant at the science center keeps making them get checked. THE EYE then needs the Horsemen to realize that the disc is fake, so that they go forward with trapping Walter in the finale. Them thinking that it is fake is the whole reason they decide to trap Walter. So the plan from the beginning of the movie is for THE EYE to prevent Walter from getting the disc. They then take this opportunity to turn over control of THE EYE to Dylan, and welcome the Horsemen in. Thaddeus was leader of THE EYE all along.

The Li from the Magic shop was the guard walking behind the Four Horsemen just before they are passing by the security metal detector in the science building. So presumably Li had stolen the real chip before them, as he is part of the EYE community...along with the other guys.

Ok this sounded interesting but it's completely wrong. Li isn't one of the guards, the Macau Science Facility maquette appears at the end in the Eye Headquarters, it was all a setup. Walter "proves the chip" in a computer that was part of the whole stage from the horsemen, they didn't have a laptop when they all entered the staged fake plane.

Octa was a setup, the Macau Science Facility was a setup, (you can see the maquettes at the final scene). The eye created all this reality to test the horsemen and setup Walter and his father.

The chip was fake all the way. Yes, its the only possible way that Walter pretended it to be real. Here is what wiki has to explain:

The Horseman infiltrate the facility and steal the chip, evading the authorities, Atlas waits in a marketplace in order to give the chip to the Eye. He is then confronted by Mabry and his men, Mabry revealing that Atlas had been fooled into thinking that Mabry was The Eye.Rhodes intervenes and pretends to retrieve the device, slipping the device back into Atlas' jacket

And once they rescue Rhodes:

After the rescue they find that the chip they had stolen appears to be a fake.

As you can see they have planned this very carefully. Once he confirm its real, they were going to thrown out of the plane as for the last scene. Where the stage was set and horsemen disclose their identity.

If you ask why he pretended, then there are many possibilities, as you can say they were drugged in the wine which they were drinking. Or they were hypnotized all the time. It can be all possible as the plane in which they were was built by the horsemen and all illusion can be created on magic stage.

Wow. Talk about over complicating things. A lot of the "answers" are plausible; that is just people trying to make up congruency. It was a long con. Think backwards instead. They all said "it was real?" (with intentionally bad acting). It was also verified by Radcliffe, so at this point, we know it was real. Backtrack to the dock where Jack says it's a fake. At this point I felt the storytelling was weak. That was purely for the audience, and for those of us paying attention, it made no sense. Backtrack further to where they steal the card but the diagnostic checks were all passing with a hairbrush inserted. That's because the authenticator was the con man in one of the final scenes. So, it was a long con from start to finish. Quit trying to find some hidden meaning. The storytellers just made a mistake in their telling.

I think it's a lot easier, without Walter pretending anything.

In the end it is revealed that everything in the showdown starting from being put into the truck has been faked and controlled by the horsemen. So it is "their plane", and it is their "chip test device", making the fake chip look real so they get thrown out of the plane (not being needed anymore and knowing too much), documenting the willingness of the "bad guys" to take part in premeditated murder. So the chip itself was still fake, but the device told Walter "It's genuine! Yay!". So why did they pretend to be surprises at it being real? That's the game inside the game: Walter and the others would have expected them to try a con using a fake (and knew that the horsemen and Dylan would not let each other be harmed), so the "being really surprised" when the chip that was handed over was confirmed "real" helped lessen suspicion. It took me a bit to figure this one out, too :).

The real bad thing is: if they stole a fake chip right from the start, then where is the real "immeasurable-danger-to-humanity-and-freedom-device" now? We could assume that "the other Eye guy" showing up in the library in the end had opportunities enough to grab it, but who knows...

Given the fact that the four horsemen were so surprised when the chip tested positive, even though the bad guys all along believed them to have it, I find it highly unlikely that they built a false 'chip test device'.

I agree partially with user40779 however I believe it was probably the man (I can't recall his name) that was overlooking the chip when the horsemen originally stole it. He was revealed to me a member of the eye at the end of the movie and could have easily stolen it before the horsemen arrived having prior knowledge of their plan from what the grandmother in the magic shop overheard of Atlas's phone conversation. Supposedly all for the purpose of preventing it from falling into the wrong hands. From that point he handed it over to li and his grandmother for them to switch it with the fake and precede to the main event.

At least that's the best I can come up with.

This is a lot simpler than everyone is making it. The chip was always fake. When Mabry tests it he says it’s real but remember, it’s a fake plane built by the eye. Every piece of equipment on the plane, including the piece of equipment Mabry uses to test the chip are fake. The eye programmed that piece of equipment to analyze the fake chip and find it to be real.