When using a comparator to power a solid block, outputting the lowest signal possible (signal carries only one block before stopping), with a piece of redstone dust connected to that block, and then another piece of dust on a vertically adjacent block (another solid block one unit across, and one unit above) with a repeater on the floor level trying to pull power out of the solid block, the repeater will not be able to get the signal.
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Duplicate of MC-8645, please use the search function to see if your bug has already been submitted. Currently over 50% of tickets are being closed as duplicate.