This page has been replaced to answer questions as late as 2012. Some points on time travel theory have been removed as simple, but fun, speculation.
Some people say they prefer a simple solution…Abe does this; Aaron does that; the end. Maybe that is good enough for some people, at least they feel they got it and understand it and off they go feeling good again. That theory though, has a lot of errors and oversights. Can the average viewer see it? No. However, if the solution was that simple, why would such debates have raged for years after the film’s release? And if the complex solution is inaccurate, why did the battle end so quickly?
I feel it comes down to two different approaches. One person buys everything at face value. This person reads a simple solution and adopts it as his own, often saying that this was the solution that was in his head already. This kind of person listens to the commentary (though extremely sparse in Primer) and accepts anything that disagrees with that solution as a simple director’s error. Example: running Abe is just an editing error, there is no Abe running around 90 seconds besides Aaron’s house before Abe gets out of the car. Artistic license or foreshadowing some will say, or a simple error. The complex approach accepts the Abe we see and calculates how this occurred. It doesn’t look to Carruth to answer and explain this point. Shane took almost a year to edit this. Do you really think he wouldn’t have noticed Abe running around a minute too early?
The solution would seem to be that Abe and Granger set out to go back a day in some urgent need. When they meet the earlier version of Abe however, he doesn't get back in the box and return as the same Abe. Rather, he gets in his own fail-safe, permanently altering events.
Another question lies in the sound editing. Perhaps you noticed that after the outdoor scene at the airport ( sitting at a Sonic ) that their is a continuation of the sound of airplanes continuing for several minutes. This was deliberate. It doesn't seem to play into the plot however. Also, some asked about the sound of the beach during their time travel. Again, it doesn't seem to be significant.