My ancient Mac Mini (2012, running Siera) has been visited by a ghost.
Its keyboard locked on repeating the letter r in all windows in which typing is possible and also in combination with Cmd- ,
while rattling loudly. Restarting didn't cure it.
But it is running headless and without a keyboard attached!
(or even a wireless one anywhere near)
There is only an external disk on the USB ports.
I verified that it is not the MacBook
used to access it in Screen Sharing being stuck by trying several.
Unfortunately I destroyed the phenomenon by turning the Mini off
over lunch.
Does anyone here have any idea about what may have been causing this?
Jan
My ancient Mac Mini (2012, running Siera) has been visited by a ghost.
Its keyboard locked on repeating the letter r in all windows in which typing is possible and also in combination with Cmd- ,
while rattling loudly. Restarting didn't cure it.
But it is running headless and without a keyboard attached!
(or even a wireless one anywhere near)
There is only an external disk on the USB ports.
I verified that it is not the MacBook
used to access it in Screen Sharing being stuck by trying several.
Unfortunately I destroyed the phenomenon by turning the Mini off
over lunch.
Does anyone here have any idea about what may have been causing this?
On 16/02/2026 21:29, J. J. Lodder wrote:
My ancient Mac Mini (2012, running Siera) has been visited by a ghost.
Its keyboard locked on repeating the letter r in all windows in which typing is possible and also in combination with Cmd- ,
while rattling loudly. Restarting didn't cure it.
But it is running headless and without a keyboard attached!
(or even a wireless one anywhere near)
There is only an external disk on the USB ports.
I verified that it is not the MacBook
used to access it in Screen Sharing being stuck by trying several.
Unfortunately I destroyed the phenomenon by turning the Mini off
over lunch.
Does anyone here have any idea about what may have been causing this?
Jan
I asked Gemini:-
Hello Jan,
That is a classic "ghost in the machine" scenario. Since you've ruled
out the client machines (the MacBooks) and confirmed no physical or
wireless keyboards were attached to the Mini, we have to look at what
was actually plugged into those USB ports and how macOS handles input.
Given your setup, here are the most likely culprits for a phantom r key:
1. The "Ghost" HID Device
Even if you don't have a keyboard plugged in, macOS can sometimes misidentify other USB devices as Human Interface Devices (HID).
The External Disk: You mentioned an external disk on the USB ports. If
the bridge chip in the disk enclosure is failing or has a "noisy" power supply, it can send erratic signals over the data lines.
The "Rattle": The rattling you heard is likely the macOS "Alert Sound"
(like Funk or Basso) playing at maximum repetition speed because the
system thinks a key is being held down.
2. Screen Sharing Artifacts
While you tried several MacBooks, the "stuck" state might have been server-side (on the Mini) rather than client-side.
If a Cmd-R or R signal was sent once during a momentary network lag, the Mini's internal HID driver might have "latched" that state.
In Screen Sharing, there is a software-level keyboard driver. If that
process (screensharingd) hangs or encounters a buffer overflow, it can
repeat the last received scan code indefinitely until the process is
killed or the machine is rebooted.
3. Logic Board / USB Controller Noise
On a 2012 Mac Mini (now 14 years old), hardware degradation is a factor.
Electrolytic failure: If the capacitors near the USB controller are
failing, they can cause electrical noise. In some cases, this noise is interpreted by the OS as a low-level interruptrCoessentially a "phantom" keypress.
The "R" key specifically: In ASCII/HID scan codes, r doesn't have a
uniquely "noisy" bit pattern, but if the system was attempting a
recovery or disk operation, certain shortcuts involving R might have
been triggered by the faulty external disk connection.
How to Diagnose if it Returns
If the ghost reappears before you decide to retire the old soldier, try
these steps:
Unplug the External Disk: If the typing stops immediately, you've found
your culprit (likely a failing cable or enclosure).
Check Terminal: Open Terminal and type hidutil list (though this may be limited on Sierra). It will show you what the OS thinks is connected.
Check System Logs: Open Console.app and look for IOHIDFamily errors.
This will tell you if the kernel was struggling with a "ghost" input device.
J. J. Lodder <nospam@de-ster.demon.nl> wrote:
My ancient Mac Mini (2012, running Siera) has been visited by a ghost.
Its keyboard locked on repeating the letter r in all windows in which typing is possible and also in combination with Cmd- ,
while rattling loudly. Restarting didn't cure it.
But it is running headless and without a keyboard attached!
(or even a wireless one anywhere near)
There is only an external disk on the USB ports.
I verified that it is not the MacBook
used to access it in Screen Sharing being stuck by trying several.
Unfortunately I destroyed the phenomenon by turning the Mini off
over lunch.
Does anyone here have any idea about what may have been causing this?
My guess is that you sent the 'R' key-down event over screen sharing (ie VNC), but then lost the key-up event. Perhaps screen sharing disconnected
at an inopportune time. That means the Mac thinks the R key is still being pressed, hours later.
When you said restarting didn't cure it, did you mean you restarted the
Mini, or the Macbook? I'd have thought a restart of the Mini would cure it.
David B. <David@hotmail.co.uk> wrote:
On 16/02/2026 21:29, J. J. Lodder wrote:
My ancient Mac Mini (2012, running Siera) has been visited by a ghost.
Its keyboard locked on repeating the letter r in all windows in which
typing is possible and also in combination with Cmd- ,
while rattling loudly. Restarting didn't cure it.
But it is running headless and without a keyboard attached!
(or even a wireless one anywhere near)
There is only an external disk on the USB ports.
I verified that it is not the MacBook
used to access it in Screen Sharing being stuck by trying several.
Unfortunately I destroyed the phenomenon by turning the Mini off
over lunch.
Does anyone here have any idea about what may have been causing this?
Jan
I asked Gemini:-
Hello Jan,
That is a classic "ghost in the machine" scenario. Since you've ruled
out the client machines (the MacBooks) and confirmed no physical or
wireless keyboards were attached to the Mini, we have to look at what
was actually plugged into those USB ports and how macOS handles input.
Given your setup, here are the most likely culprits for a phantom r key:
1. The "Ghost" HID Device
Even if you don't have a keyboard plugged in, macOS can sometimes
misidentify other USB devices as Human Interface Devices (HID).
The External Disk: You mentioned an external disk on the USB ports. If
the bridge chip in the disk enclosure is failing or has a "noisy" power
supply, it can send erratic signals over the data lines.
The "Rattle": The rattling you heard is likely the macOS "Alert Sound"
(like Funk or Basso) playing at maximum repetition speed because the
system thinks a key is being held down.
2. Screen Sharing Artifacts
While you tried several MacBooks, the "stuck" state might have been
server-side (on the Mini) rather than client-side.
If a Cmd-R or R signal was sent once during a momentary network lag, the
Mini's internal HID driver might have "latched" that state.
In Screen Sharing, there is a software-level keyboard driver. If that
process (screensharingd) hangs or encounters a buffer overflow, it can
repeat the last received scan code indefinitely until the process is
killed or the machine is rebooted.
3. Logic Board / USB Controller Noise
On a 2012 Mac Mini (now 14 years old), hardware degradation is a factor.
Electrolytic failure: If the capacitors near the USB controller are
failing, they can cause electrical noise. In some cases, this noise is
interpreted by the OS as a low-level interruptrCoessentially a "phantom"
keypress.
The "R" key specifically: In ASCII/HID scan codes, r doesn't have a
uniquely "noisy" bit pattern, but if the system was attempting a
recovery or disk operation, certain shortcuts involving R might have
been triggered by the faulty external disk connection.
How to Diagnose if it Returns
If the ghost reappears before you decide to retire the old soldier, try
these steps:
Unplug the External Disk: If the typing stops immediately, you've found
your culprit (likely a failing cable or enclosure).
Check Terminal: Open Terminal and type hidutil list (though this may be
limited on Sierra). It will show you what the OS thinks is connected.
Check System Logs: Open Console.app and look for IOHIDFamily errors.
This will tell you if the kernel was struggling with a "ghost" input device.
Thanks. Ill keep it in mind for the ghost's reappearance,
Jan
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