Tim Coe's description:
The dominant operating mode of line puffers is period 4. The immediate exhaust alternates between a 2 X ~N block of on cells followed two generations later by a 4 or 5 X ~N block of on cells. Occasionally this phase reverses itself. During the process of reversal part of the line puffer is operating in one phase and part of it is operating in the other phase. In both of the failures that I have currently observed the line puffer decay was associated with the defect between the two phases propagating to the edge. You can see the phase reversal begin at roughly generation 2600 of the pattern that I included.... One of the defects propagates to the edge resulting in engine failure 800 generations later.
You won't see the disaster until generation 3460 or so. This will take a long time in the applet. I recommend downloading the pattern to view off-line.