Hi there,
So, yeah, the title sums it up pretty well. Sometimes, repeaters don't unpower and keep emitting a signal even though they are not receiving any signal.
A world download with several examples of when this bug occurs (along with my very unrequested opinion on the matter) has been attached to this bug report (
[media])
If you are just way too lazy to download the world, here is what is happening: when you synchronize the powering and unpowering a repeater with it powering and unpowering something, it will not update and thus will not unpower. For instance, you can put an observer and a repeater with delay one behind it and, when you put a block in front of the observer, the repeater will output a constant signal until it is updated.
Obviously, what I expected to happen was for it to turn off.
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Duplicate of MC-151057

I beg your pardon, but, in my opinion, even though, those two are very related, neither of them is a duplicate of the other...
They have the same cause, so they're essentially the same issue. I've updated the title of the linked ticket a bit to better reflect that not only repeater clocks are affected.