The latest mechanics implemented seems to complicate the process a bit in creative (see http://mod-buildcraft.com/forums/index.php?topic=370.0). We probably need to force a bit more the sequence. Make sure that solid blocks are computed separated from non-solid blocks by sorting them separately, maybe refuse to build blocks before there's still things to clear, and non-solid blocks before there's still solid block to clear. If things are ordered properly, this can be checked by comparing the current schematic with the head of the list.