Comment 8 for bug 55843

After some experimenting.
Disabling usbhid/hid and using usbkbd/usbmouse did not help.
When problem occurred, there was no info in ~/.xsession-errors or the Xorg log file or dmesg.

While the problem was occurring, unplugging the USB keyboard and plugging it back in, or loading/unloading hid/usbhid did not halt the error.

I suspect then the flaw is in X - in keyboard repeat I guess.
Holding down the key while clicking on mouse befuddles its simple brain?

Will look for similar X errors.