In the previous snapshot, 16w41a, observer blocks picked up the output of command blocks like a redstone comparator, but did not 'freeze' like a comparator does when the command block does not update (like those of redstone activation mode only commands) thus creating a 'forever on' comparator which 'freezes' many on-off command lines.
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Will add to MC-107664.
The observer block used to output a signal when a command block completed a command like a comparator normally does, but just outs a pulse like it should. Now it does not at all in this recent update.