Although redstone circuits were supposedly made more reliable a few snapshots ago, I still have this odd intermittent problem with the drop-pusher at the bottom of my XP farm's fall-shaft: of the two blocks pushed forward two blocks to make drops accessible, only one of them advances fully. This only occurs after the chunk has freshly loaded (generally due to restarting Minecraft, or due to returning to the area via Nether portal). The redstone paths powering the pistons on either side are essentially identical, but the two sets of pistons clearly don't act in concert.

Stranger still, operating any other nearby redstone circuit (either the lighting system in the mob-spawner darkroom above, or the bookshelf-moving pistons in my nearby enchanting room) will make THIS circuit behave correctly until the next time the chunk is reloaded. The two recordings in the video below were made about one minute apart; all I did in between was switch a few pistons on and off. The circuits are about 11 blocks apart, so it's not a block update thing.

I can't be sure this isn't a duplicate, since so many redstone bug issues are summarized simply as "Redstone!" or something similar, and most of them aren't actually issues. If necessary I'll collect images of the circuit in question, but it's a bit convoluted and ad-hoc.

Youtube video showing error: http://youtu.be/U-dNFyGH1QA