If you break a block below the block the detector rail is on, and place a redstone dust on it, the redstone dust is powered, but the redstone dust on the same level as the detector rail is not powered. (Minecart is not on the detector rail)
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24th duplicate of MC-5788 , please use the search function to see if your bug has already been submitted. Currently over 40% of tickets are being closed as duplicate.