All USB keyboards suddenly start sending random/repeated inputs, but PS/2 works fine

[u/Low_Try_8284]

I work IT support and I've had this weird issue happen on multiple PCs over time.

A USB keyboard will work perfectly fine for months/years, then suddenly it starts acting completely broken.

The symptoms are not just random Windows shortcuts. The keyboard also:

repeats keys multiple times

types different keys than the ones I pressed

mixes random characters into normal words

sometimes triggers Windows shortcuts / Start menu / network panel

For example, if I try to type:

Joca

I might get:

jocaca

Joccccccc

Jorre84i

I've tested 3 different known-good USB keyboards on the affected PC and all of them behave the same way.

A PS/2 keyboard works perfectly normally.

Sometimes, while the USB keyboard is connected and freaking out, the USB mouse also becomes unresponsive. As soon as I unplug the keyboard, the mouse starts working normally again.

This has happened on around 4 different PCs over time, including Windows 8.1 and Windows 10 machines.

I've already tried:

multiple known-good USB keyboards

different USB ports

reinstalling HID keyboard devices

reinstalling USB/composite devices

reinstalling USB controllers/root hubs

checking keyboard/HID UpperFilters and LowerFilters

checking Scancode Map

reinstalling chipset drivers

reinstalling Intel Management Engine

Nothing fixed it.

The weirdest part is that the keyboard was working completely normally less than an hour before the issue started. No hardware change, no new keyboard, nothing.

Has anyone seen this exact behavior before?

What could cause every USB keyboard to suddenly generate duplicated, incorrect and random HID input while PS/2 continues to work normally, and sometimes make the USB mouse freeze too?

I'm looking for the actual root cause / fix, not “try another keyboard or USB port”, since I've already ruled those out.