Ask anyone about the Kessel run, and everyone knows Han Solo did it "in less than twelve parsecs."
Dig a little deeper into the mythology and you might find references to the run being through the Maw Black Hole Cluster, or claims that Luke Skywalker and Kyp Durron finessed their way through using the Force, but it's uncontested that the speed record belongs to Han, or at least he claims it does, and I'm not going to be the one dissing the guy with the itchy blaster finger...
But the infamous "Kessel run" line in Star Wars is also a classic and often-used example that's emblematic of a larger problem in the popularisation of science.
The problem I am referring to is that many concepts in science are counter-intuitive, and many words in the language of science are used quite differently from their everyday usage. Compounding the problem is the fact that words often have different meanings in different branches of study. Consider, for a simple example, the word "cosmology" and the different meanings it has in astronomy and philosophy.
What happens then, in many cases, when a scientific concept "breaks out" into the "real world," or even into fiction, is that a half-remembered explanation, often from a well-meaning media source, is combined with common knowledge to create something quite wrong, which is then applied to other related knowledge to completely muddy the waters.
One word (or hyphenate) that has been particularly prone to this is "light-year". Of course, it's a unit of distance (how far light travels in a year), but because of the "year" tacked onto the end, it's often thought of as a unit of time. It would indeed take a long time to travel one ly at currently achievable speeds, but that's neither here nor, sadly, there.
Now, a parsec is also a unit of distance. It's a fascinating and useful unit, and a cool-sounding word. Parsec! And because it's longer (see how these confusions occur? curse you, English!) that leads to the misconception that it's more important. I have actually read in popular physics books that the parsec replaced the light-year because the latter was too short! That is such an odd (and wrong) notion, especially when you consider the relationship between the two (a parsec is about 3.26 ly) - if one was looking for a "new/super/better" astronomical unit, wouldn't a more sensible choice be something one hundred, or one thousand times longer?
But back to Han and the Falcon. Good old George Lucas, sitting down and writing the script for the movie, makes the classic error of thinking that a light-year is a unit of time, has heard that a parsec is the same thing only more "futuristic", or equates the "sec" part with second (microsec-ond, kilosec-ond, parsec-ond? cool!), and so has Han boast about making a "run" (clearly a distance) in less than "12 parsecs" (clearly a time). Or some other even more innocent and/or bizarre mistake.
No amount of retroactive continuity (or "retcon") by fans can fix what is clearly an error. One says "I made the morning run into work in less than 25 minutes." And since all of Lucas' other characters speak in 1970s American English, one cannot, honestly, hold this sentence out as the only exception in an otherwise completely vernacular script. The litmus test must be the context in the script itself:
Obi-Wan Kenobi: Yes indeed, if it's a fast ship.
Han Solo: Fast ship? You've never heard of the Millennium Falcon?
Obi-Wan Kenobi: Should I have?
Han Solo: It's the ship that made the Kessel run in less than twelve parsecs!
Fast, of course, means that something has impressive speed. Speed, for those following along with the study guide at home, is distance (the "Kessel run") over time (those twelve "parsecs").
In the same way, nonsense phrases like "we've made a quantum leap in our research" or "we're light-years ahead of the rest" also illustrate the point, and although the latter can be innocent, it rarely is. Nothing seems to have quite the same sting as the Kessel run, though, because none of those lines were uttered by someone as downright cool as Han Solo.
Particular thanks to http://www.badastronomy.com/bad/misc/light_years.html , and several excellent writeups on this site.
Also thanks to dem bones for helpful editorial comments, and the dozens of worthy noders who pointed out the weakness of a particular generalisation (you know who you are and you know what it was!)