The one for Primer:

The ending scene at the airport is the only shot involving "time travel" that is from the original storyline. The events leading to this point are lost to the revisions from the failsafe box.

Abe did not create the original failsafe intentionally, the original box wasn't intended as a time-travel device, but an exploration of the effects of the box on a human. Abe suffers from OCD (director's commentary) and would prefer to test it on himself before allowing anyone else to do so. He discovers the time-travel function during his self-testing.

At the end of the movie, we hear him comment that the box he was building wouldn't work because it was wired wrong, and then he promises to continue sabotaging his past-self's progress as long as he can.

Something horrible happened, probably Rachel being killed at the party, and Aaron became so insane attempting to correct the event that Abe decided he had to be stopped.

Abe from the original storyline (hereafter Abe0) ends up using the first box, which had been left running as a test, like a failsafe to undo all the drama. He then sabotages it so Abe in the movie storyline (hereafter Abe1) believes he still has not corrected the flaws. His intention is to continue sabotaging the boxes until Abe1 gives up hope.

Abe0 then builds a new failsafe so he could have independent control over his double (the Abe we see in the movie)

Abe1 discovers his box is broken, builds a new box. The is the box he eventually shoes Aaron in both timelines.

Aaron in the movie storyline (hereafter Aaron1) discovers Abe0's box in his own timeline, believes Abe1 has lied to him, failsafes carrying a box through with him.

Aaron1's box is actually subject to Abe0's failsafe.

Abe1's failsafe is actually subject to Aaron1's failsafe.

There are two causal loops, and wherever Abe0 is, he is the only person who knows the original story. Although he is capable of ending the loops, he will not, as he requires the running failsafe to prevent whatever catastrophe caused him to take the leap-of-faith failsafe in the original storyline.