First off, two things: the movie Gravity is likely the most realistic space travel-based movie ever made, and I'm about to spoil the crap out of it by revealing one huge flaw. So, if you don't like spoiled movie, you may want to leave. Okay, there's your fair warning. Everyone else, come along!
Now, there's already been a number of sites pointing out flaws in the science of the movie, and while it's incredibly accurate in many ways, there are some issues. Various sites have pointed out the phantom magic hatch on the Soyuz' re-entry module (though, to be fair, it does match with one on the Soyuz simulator), the inaccurate orbital locations of the three main locales of the movie (Shuttle/Hubble, ISS, and a hypothetical future version of Tiangong), and, most importantly, the fact that under a spacesuit astronauts wear bulky cooling garments and a diaper, not sexy athletic-gear underthings. Unless you think adult diapers are sexy.
The biggest error, though, isn't one I've seen talked about very much, but in some ways it could be the one that, if corrected, would change the movie most dramatically. Because correcting it would effectively remove Sandra Bullock's chance of survival, which would take the speaking characters' survival rate in the film down from 33% to 0%.
The big error is this: if both the ISS and China's space station were, as the movie states, evacuated soon after the debris cloud impacted the shuttle, no rescue/reentry vehicles would be still docked to their space stations.
To put it simply, the last surviving astronaut, Dr.Ryan Stone, would have had no ride home. As the movie stands, she has two unused re-entry capable spacecraft to choose from, which makes no sense at all in the context of the movie.
Here's how the capsules used to ferry crews to space stations are generally used. I'll use the Soyuz/ISS model as an example, but the China's Shenzhou should be effectively the same, as it's based on the Soyuz model.
A Soyuz spacecraft is launched with a crew of three to rendezvous and dock with the ISS. This is how crews actually get to the ISS, and is the only way now that the Shuttle is retired. The shuttle could bring crews up as well. As the ISS in its current (and as shown in the movie) configuration has a crew of six, two Soyuzes are used to bring up crews, each one carrying a maximum of three passengers.
A Soyuz has an on-orbit lifespan of about six months, so, generally, when either crew replacements or a visiting crew is sent, they go up in a fresh Soyuz and return in the old one, insuring that the Soyuzes docked to the ISS are always within their operating parameters. With six people aboard, that means there are always two Soyuzes docked at the station, ready to drop back to earth in the event of emergency.
At the Chinese Tiangong station, which appears to be built to the configuration China is planning for 2020, there would be three Taikonauts aboard, which means they'd only need to keep one Shenzhou return ship docked, which would be the one they came up in. Which is exactly what we see in the movie. So, if the Shenzhou is still there, how did the Chinese crew evacuate?
The same question goes for the ISS — if there was a full crew of six aboard (there's no reason to think there wasn't) how did they get back? One Soyuz is gone, so that accounts for three, and the other Soyuz is damaged — debris impacts broke open the parachute container, so it wouldn't have been usuable for re-entry anyway, and would have been left — but that doesn't account for how the remaining three astronauts and/or cosmonauts got back to Earth.
It would be extremely difficult to cram in six people into a Soyuz return capsule — with only three seats and very little room, it's likely impossible. Perhaps there was another shuttle docked to the station, that could have taken home the remaining three, but that usually has to be planned for before to provide safe seating in the shuttle orbiter's mid-deck.
All manned Soyuzes and Shenzous need to be piloted up by people. Technically, they could be sent unmanned, as Russia does with their Soyuz-derived Progress freighters, but so far this has never been done to the ISS (though it had been done back in 1979, to Salyut 6). It could be theoretically possible for an extra Soyuz to be parked at the station, as there are four possible docking ports for a Soyuz.
In the movie, one of these ports, the one at the aft end of Zevezda, the Russian main living module, is taken up by an ESA Automated Transfer Vehicle, and the other one used is the docking port on the Pirs module, which is where the damaged Soyuz is. That means there would be two free ports for the two other Soyuz spacecraft.
Even though there's enough docking ports, two of those ports are usually taken up by Progress resupply ships and not some empty Soyuzes, which, as we said before, have never been launched empty and unmanned to the ISS. Why would they keep an empty, unused Soyuz docked, taking up a valuable docking port? The answer is they wouldn't, and they don't, mostly because it's too expensive to launch one up empty when it could have up to three paying customers aboard.
Are there any ways around this? Could half of the ISS crew have already been picked up by the shuttle servicing the Hubble? If the movie world is like ours — which it basically is shown to be — no. Shuttles sent to service the Hubble have their cargo-bay mounted docking equipment removed. A Hubble-service Shuttle actually can't dock to the ISS.
Maybe some near-future regulations will demand an empty, spare Soyuz be docked to the station? And the Chinese will require the same thing (with a Shenzhou) for their station? Not likely, because, as I mentioned, those things cost money, and would never really even be used.
So, where does that leave things? If the movie was actually accurate to how things are really done, at least one Soyuz would be returning to Earth, and the damaged one would be used as a lifeboat by the remaining three, who would have taken it to an orbit away from the debris cloud and would be awaiting a rescue mission.
Sandra Bullock's character would have still been wise to try and get to the ISS, at least to resupply her oxygen and maybe grab a snack or something. In the movie, the ISS was still in the same orbit as the debris cloud, so she wouldn't have wanted to stay long, but getting to the Chinese station from the ISS, while technically possible in a Soyuz, wouldn't help, since the three Chinese crew would have already used their Shenzhou to get the hell out of there.
So, what could she have done? Based on the hardware we see in the movie, she would have had one possible option: the European ATV. It's the only independently-flyable part of the ISS outside of the Progress and Soyuz ships, it has power and is pressurized so she could, theoretically, live in it a while.
Still, pressurized doesn't mean full life support, so she'd have to go in there in a space suit, and bring some extra oxygen — though its possible the ATV was bringing oxygen, food, and other supplies in it already. It could probably be made habitable, if she uses her space suit to keep her warm if needed, and manages to find a way to scrub the CO2 out of the air.
The ATV has the means to travel to a safer orbit, but no on-board controls, nor any means to re-enter the atmosphere without becoming a fireball. It's automated and/or remotely operated. There may be a remote operation console on the ISS she could take and use inside the ATV, maybe. Maybe there's a way to get some manual overrides from inside the ATV? I honestly don't know. If communication with Earth could be established, ground control could possibly send her to a safer orbit to await a rescue Soyuz, but that's all very iffy.
I get why the decision was made to have some re-entry vehicles still up there — you lose most of the plot of the movie if you don't give the hero at least some possibility of getting back home. And, all in all, Gravity is an incredible movie, and the physics and space hardware are recreated with loving, obsessive attention to detail.
But, this is still a pretty huge inaccuracy, and while it in no way spoils the movie, it's worth pointing out. Just so you know what to expect if this ever happens to you.