In the movie Independence Day, alien invaders are successfully repelled when plucky geek Jeff Goldblum uploads a computer virus to the alien mothership, disabling the attacking ships' protective force fields and allowing the combined military of the entire world to take them all down at once.
Goldblum's character does this using an Apple Macintosh Powerbook 5300.
The suggestion that the operating systems of an alien mothership over 500 kilometres wide, thousands of years more advanced than anything humanity has yet created, large and sophisticated enough to support an entire alien civilisation, would be compatible with Mac OS is regarded by the majority of commentators as one of the most egregious misrepresentations of computing technology in cinematic history.
I, however, disagree.
Why it could work
In the movie, it's revealed that first contact with the aliens occurred in 1947 with the interception of an alien scout ship over Roswell, New Mexico: the Roswell incident. Alien technology does not advance significantly over the intervening 50-odd years which makes the downed advance scout identical to the fighters deployed during the main invasion. That means its systems are still compatible with those of the alien mothership. Evidence for this is provided when Brent Spiner's character explains that following the arrival of the mothership, all the subsidiary systems of the scout have been automatically reactivating themselves.
This is not to say that the scientists at Area 51 responsible for studying the scout had no success activating - or at least studying - the systems by themselves. Having had 50 years to investigate the scout's onboard systems it is entirely plausible that they would have figured out roughly how they should work - if the interfaces are reasonably simple then they could have done this without even activating them. The mothership wasn't around so the scout remained resolutely dormant in the absence of some sort of wake-up code which the scientists never figured out, but even so, it seems preposterous that after decades of work they would NOT have been able to jury-rig a pretty sophisticated set of hardware adapters and software interpreters to allow their computers (presumably Apple Macs) to interface with the scout.
Now, we know alien scouts are usually deployed by the City Destroyers which means they must have some capability to pass through the Destroyers' shields - either slipping through them by modulating their own onboard shields, or (more likely) by interfacing with the Destroyers' shield systems to open gaps as and when required. So we can surmise that alien fighters have at least some level of access to the Destroyer shield systems. Therefore, so do the scientists. (They possibly never realised it, due to the absence of any Destroyers for them to remotely interface with via the downed scout during the 50-year gap.)
Also bear in mind that simply being staggeringly advanced technologically does not guarantee that your software will be impregnable. In fact, the more complex the software, the more likely it is to have holes in it, and this software runs an entire civilisation. These aliens could be using the cosmic equivalent of Microsoft Outlook Express for all we know. Also, there is no evidence to suggest that they have ever had to deal with anything other than direct physical assaults - hence the lack of security in their shield protocols and the intrinsically weak single-point-of-failure network structure (even if said point of failure is a gigantic near-indestructible spacecraft).
The final piece of the puzzle is Jeff Goldblum's character. He is a "TV repairman" but clearly significantly more technically able than this title would suggest. He is MIT-educated, able to craft and use directional microphones, and, most importantly, has had direct experience of the aliens' internal communications protocols, having successfully decoded their strike countdown from TV satellite signals on July 2nd. He has a Macintosh of his own... and, presumably, extensive programming/hacking/phreaking experience. He has a brain.
Now let's paint the picture. Late at night on July 3rd he randomly stumbles across the virus idea. He assembles all of the above facts in his head - most likely, in a matter of seconds - he pulls up the Area 51 Mac OS/alien OS interface suite, plugs his Powerbook in, and puts in the hardest night's work in history, discovering a security hole first in the scout's shield systems and then in the entire alien shield network. (The first security hole may possibly have been discovered by the scientists before him, but it is certainly he who first figures out a way to deploy it fleet-wide.)
Of course, the shields will only be deactivated briefly before the alien generals on the mothership realise what has happened and reactivate them manually, so the nuke has to be placed at the same moment as the virus is deployed into the heart of the mothership and something something worldwide defence forces blah blah blah... you know the rest.
The key link in the chain is the pre-existing interface technology. Once you have that, the only major hurdle that your suspension of disbelief must overcome is Jeff Goldblum's superhumanly fast coding skill. Perfectly within the bounds of reason - at least, for Hollywood.
It is admittedly preposterous that the aliens would let a poorly-flown, uncommunicative, long-lost scout ship right into their lair... or that a nuclear bomb of ANY size could completely annihilate an object with 1/4 the mass of the Moon... or that you can escape from a ship 550 kilometres in diameter in 30 seconds without being crushed by acceleration... and so on. We knew all this from the start. Independence Day is still a ridiculously stupid movie.
But. BUT. This particular objection, this Mac/alien incompatibility thing - I don't buy it! And nor should you. You want movies with stupid computers? Have you seen the trailer for Untraceable yet? "Oh no, this mysterious website is killing people!"