When two observer blocks are placed horizontally facing each other, they create a constant redstone loop. If a piston is placed below one of the observers with an air block between them, the piston still receives the activation signal and functions as if connected.
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See MC-108.
Thank you for your report!
We're tracking this issue in MC-108, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you would like to learn more about this particular feature, see this article on the Minecraft Wiki.
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