I was trying to make an bow with command blocks that would create fire around where the arrow landed. I had some command blocks set up with these exact commands commands in this order:
/clear @p minecraft:bow 1
/testfor @e[type=Arrow]
/kill @e[type=Arrow]
/give @p minecraft:bow
The commands work, but one time only. From the testfor command onwards, the comparator and repeaters dont turn off, so the command can only be run once. I can fix it manually everytime by going into the command block and pressing "Done" and the repeaters turn off. I don't know if this was intended, but without a signal going in to it, there shouldn't be any repeaters powered.