Once the kitchen could sense itself, the physical motion sensors sitting in it were redundant. So I moved them. One went to the laundry room, one to the guest bedroom, and I renamed both to match where they had landed, which felt like tidying up.
It was not tidying up. It was a rename, and almost everything in this house is wired together by name.
What a rename actually breaks
The automations that used those sensors did not error. They did not warn me. They carried on referring to a name that no longer pointed at anything, or worse, pointed at the same hardware now sitting in a different room and reporting on a different space. The guest bedroom started being able to trigger things that belonged to the kitchen. Nothing looked broken from the app. You only find it by walking around the house at night waiting for lights that never come on.
The guest bedroom is a slightly absurd place to have put a motion sensor, incidentally, because nobody in the house has worked out what that room is for yet. It has a bed in it and a chair nobody sits in, and about once a fortnight somebody carries laundry through. I gave it a sensor because I had a spare sensor, which is not a reason. Half of this project so far has been driven by what was already in the box.
I hit the same class of bug again a few days later from the other direction, when a switch got renamed in the Home app and every bit of my own code that addressed it by name went quietly dead for most of a day. So the rule I have now is that renaming anything is a change to the system, not a cosmetic tidy, and it goes in the log with everything else.
The gate pointing at a freezer
The sensors got jumbled in the move. Three identical white pucks in a box, all with the same model name, and no way to tell from the app which one was physically in my hand. Serial numbers are printed on the back in a grey so light it may as well be decorative, and I was not going to unpair them all and start again.
So I used the freezer. Put one puck in, shut the door, and watch which sensor in the app drops to nothing. It is a sealed box with a known light level, which is more than I can say for anywhere else in this house at four in the afternoon. It worked perfectly and I felt quite pleased with myself, and I would do it again.
What I did not do is take it out promptly. It sat in there for the better part of two days while I got distracted by the switches upstairs. Several of the automations are gated on ambient light, so they only fire when the room is dark, and that gate reads the level from a named sensor. A light sensor in a closed freezer reports darkness confidently and consistently. Every automation gated on it believed the whole house was in permanent night. It took an embarrassing amount of time to find, because everything downstream was behaving exactly correctly given the input it had, and because I had stopped thinking of that puck as a sensor and started thinking of it as a thing in the freezer.
The one that was pretending to be dead
While cleaning up I found an old automation left over from the first week. It pointed at two stair sensors that had never successfully joined the new bridge, so it could not fire, and it had been sitting there looking harmless. It was not harmless. The moment I got round to re-pairing those sensors, which is on my list, it would have woken up and started recalling a scene in the wrong bedroom on every trip down the stairs.
I deleted it, along with an orphaned sensor group it had left behind. There is a general lesson in there about things that look inert only because their inputs are currently missing, and I do not think I have fully absorbed it yet.