Han Solo's line about the Millenium Falcon making the Kessel run 'in less than 12 parsecs' might be completely wrong
The Kessel Run is a fictional Star Wars route that smugglers use to get past the prison planet of Kessel. Saying that the Millenium Falcon can make it in 'less than 12 parsecs' implies that parsecs are units of time. In reality, a parsec is a real unit of distance! A parsec is 3.26 light years.
While movies tend to have a number of factual or continuity errors in them, this one is a little special. It has been the subject of a lot of discussion. As a result, there are a number of theories as to what Han Solo actually meant. For example, in one of the drafts of the original script, Ben Kenobi is described reacting "to Solo’s stupid attempt to impress them with obvious misinformation," implying he knew the fact was wrong.
The other major explanation is that the fact is right, and what Han Solo is trying to say is that he the ship allowed him to map the shortest travel distances. In the audio commentary, George Lucas comments that due to asteroids, planets and other obstacles, space travel cannot be done in a straight line. This means that the fastest ships would be the ones that are able to travel the shortest distance between points A and B. The Expanded Star Wars universe clarified that there were a number of black holes through the Kessel Run, and that the Millenium Falcon was able to get really close to them, shortening the distance traveled.
Then again, it might just be a George Lucas oversight. In the novelization of A New Hope, Han Solo just says that the Falcon makes the Kessel Run in under "12 standard time units."