James W

Confirmed. Bug seems partially timing-dependent, though I haven't been able to confirm that yet. It does seem to happen when in a stack of pistons, there's upper pistons extended, and the piston on the bottom gets a signal – the piston above the bottom one disappears.

I've found that half-slabs are not even required to cause the crash. It also happens when a repeater is pushing a signal to redstone on top of a block it's facing into, and redstone on the other side of the block at the same time.

At least, that's what's happening in some of my elevator cores.

@William Pearson
Unfortunately, half slabs allowing signals to travel up to another block level in direct sequence is necessary in keeping the cores small. Also, the last snapshot that I could use such a piston device was 13w03b, but redstone characteristics changed, forcing a redesign. This version won't work on the older snapshot, and vice-versa. Makes sense that I didn't run into this bug before... https://mojang.atlassian.net/browse/MC-8198 kept me from working with the project until it was resolved.

Edit: forgot to state that all levels use half-slabs, but only a few cause the crash.

This exact crash happens randomly in my new elevator cores...the problem is that it is random. Most of the switchable circuits work without crashing, while some crash constantly, and a few others crash at times and not others. I'm using vanilla Minecraft, and the schematic file can be found at the bottom of the second post here: http://www.mcschematics.com/index.php?topic=14787.0