"Cypher" ... that word in the glossary of organ playing that's virtually synonymous with "mishap" ... is the term applied to a "stuck note" -- a defect resulting in the continuous sounding of a pipe, the key of which has not been, or is no longer, depressed.
This is a meaning that will not be found in most, if not all, dictionaries and refers to a pallet stuck open underneath a pipe, causing pressurized air to keep entering it; that particular pipe sounds and won't turn off.
Although rare, this can happen for a variety of reasons -- sometimes without depressing a key at all, as when the organ is turned on, with or without a stop being pulled.
The key itself may also stick down, in which case the cause for the cypher is more readily apparent; this may fix itself by tapping the key repeatedly.
If we play a pipe organ, especially an older, historic instrument, sooner or later it's inevitable that we're going to encounter a cypher -- and the larger the instrument, the more likely it will happen.
There are 2 kinds of cyphers in pipe organs: 1) the simple (or wind) cypher, which is generally caused by sticking or dirty or broken magnets or armatures or, more rarely, valve problems, and 2) the relay cypher, which has the same root cause and results in a continuous electric charge to one or more chest magnets, thus causing the respective pipe(s) to sound.
Aside from a potential wiring issue, organ pipes and wind chests are sensitive to temperatures and humidity -- and, right on time, with the onset of cooler weather, many of us experience a stuck note.
Sometimes we can un-stick the pallet from the console; we can try tapping the key over and over again, repeatedly sending an electrical signal to the magnet which allows pressurized air to enter and leave the chest below the stuck pipe; the pressure differential causes a pneumatic within the chest to first collapse, then recoil when the key is released, pulling the pallet open and closed underneath the stuck pipe.
We can also try mashing down a bunch of keys at once, with lots of stops drawn; this both sends a signal to the pipe and takes some of the air pressure off that particular note because it's going to a lot of other notes.
Both of these attempts speak to trying to wiggle the pallet on the stuck note, and sometimes it will work -- but, obviously, it generally cannot be done in a performance situation.
The obvious quick fix solution is: just don't use that stop ... try to find it as quickly as you can and keep its drawknob or stop tablet retired; when that isn't possible, the entire manual division may have to be ignored for the time being and some other manual(s) used.
If the ladder must come out for a trip to the inner sanctum, there are a few quick fixes ... in the case of a flue pipe, a piece of paper, dry wash rag, or towel can be gently stuffed into the mouth of the pipe, the pipe (if it's a small one) can be gently pulled out of the wind chest, or the offending pipe can be lifted out and reseated with a small piece of paper underneath, blocking the air flow.
In the case of a reed pipe, if all else fails, it may be necessary to shove the tuning wire down into the boot to close off the reed over the shallot completely, thus creating a "dead note" until the builder can come and fix the problem.
Finding the right pipe requires a little detective work, but it isn't rocket science; we can know if it's a metal or wooden pipe from the sound (timbre), and we can know it's approximate size from the octave where the cypher is located, and we will also know which divisional chamber in which to do our searching; if moving the Swell shoe affects the loudness, then the cypher is in the Swell; if moving the Choir shoe affects loudness, we should check the Choir; if there's no change in loudness, then we can suspect an unenclosed Great or Pedal; by following our ears we can thus get an idea of where in the organ chamber to look.
We try to match the pitch by playing various keys; this is done slowly so we can keep track of the right key, since tapping it may fix the cypher; when we're not sure which key we pressed, then the builder doesn't always know precisely where to look to find the problem.
We try to find the right stop, and thus the right pipe, by turning off all the stops; if the cypher goes away, we make a mental note of it; we then go through all the stops one by one to determine which one causes the fault.
If turning off the stops doesn't make the cypher stop, we can still go through the stops one by one to find which stop is cyphering.
We need to be careful not to move tuning slides while looking for a cypher; touching the top of a pipe, especially moving the tuning slide (or stopper if it is a stopped wooden pipe, cap if it is a capped metal pipe), can easily render it out of tune.
We can best find the offending pipe by blowing across the top of the pipes (blowing into a pipe causes it to stop playing), blowing into the mouth of a pipe, lightly dragging a dollar bill across the tops of the pipes (paper money is too light to move a tuning slide), or sticking a piece of paper into the pipe mouths.
There are some safety concerns if we find ourselves doing this; well meaning people can get seriously injured if they don't know what they're doing when they're crawling around an organ chamber searching for a cyphered pipe.
Firstly, we need to protect our hearing and either stuff our ears with lambs wool ear plugs or wear standard external ear protection before entering the organ chamber; big reeds on high pressure wind have a tendency to cypher more easily, and crawling around for several minutes approaching a cyphered, high pressure Trumpet, Tuba, or Bombarde pipe without ear protection can be deafening and produce permanent hearing loss.
Secondly, we need to use enough light; when it comes to searching for something, we can't do well what we can't see well.
We need also to be careful and deliberate to keep our balance on ladders and when navigating wooden passageboards; falling into a crowd of metal organ pipes from above is a good way to get impaled ... trauma and serious medical complications can result.
When we're crawling through an organ chamber we need to remind ourselves NOT to engage in stunts -- like standing on top of the largest quadrangular wood pipes nearly 40 feet above the floor to take "selfies" -- in such close quarters this can be the cause of a serious fall, and on the way down it isn't just the sudden stop at the bottom that can hurt us.
We need to wear rubber sole shoes and refrain from going into the organ chamber wearing our organ (leather soled) shoes; organ shoes are made to slide on pedal keys, not grip wooden planks or the rungs of ladders.
The organist should also be cautioned here (there are so many cautionaries in music) that if it hasn't already been done, and whenever there is time, that it's best to seek permission from the officials to enter the organ chamber before any adjustments are attempted.