When powering a repeater from the side, the back torch's texture is replaced with a strip of bedrock. Only works when the repeater is powered with another repeater.
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33rd Duplicate of MC-4449 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.

it's supposed to do that.
it's called "LOCKED" repeater. It's existed since I think 1.4