Oops, somehow that pasted in the wrong place.... The error was "The block at -326,41,-518 is air (expected: Wool)."
It's not happening anymore... Weird...
Huh, I tried using /testforblock, and noticed something not included in the post, I typed in "/testforblock -327 41 -519 35 8" and it displayed the error "The block at -326,41,-518 is air (expected: Wool)." but the air block was at -325 41 -517.
This fits in even more closely to the /setblock glitch.
Huh.. I'll try reinstalling the snapshot, maybe that will fix it.
It's the same amount off as setblock, so I think the problem in the code is the same for the two commands