If you power piston like this (picture) it does not retract. That can ruin a contraption. You can even remove all of the surrounding blocks (First other piston, then torch and then the block) and it will remain extended.
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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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Duplicate of MC-108 which was later resolved as "Works As Intended".