When I got back to my base after an extended hike in the surrounding area I noticed that 5 redstone engines was stopped dead. These were fitted right next to each other, on a straight line, each operating a wooden transport pipe in order to empty my automatic processing blocks (furnace, automatic crafting table, etc.).
They still had redstone power, and toggling the lever beneath them off and then on again did not work. I had to break them down and refit them to get them running again.
A thought struck me and I hit F9 two times to show the chunk boundary grid and sure enough, the boundary was right between the engines and the wooden transport pipes they were operating.
Could the reason for them stopping be that the chunk with the pipes was unloaded before the chunk with the engines, so that for a time, the engines thought they were powering nothing? I've read this can cause them to malfunction.
Note that I have about a gazillion redstone engines all over the base and only these 5 in a row had stopped, the rest have been running nonstop without problems since fitted.