If the Num lock key is toggled rapidly, there's a chance that mate-settings-daemon will go into runaway CPU usage (causing Xorg to also appear to be in runaway CPU usage), and the Num lock LED will flash/blink erratically (randomly--this is not the steady 2Hz blinking when a kernel panic occurs). By killing mate-settings-daemon, the system can recover; mate-settings-daemon respawns, Xorg goes back to normal CPU usage, and the Num lock LED is stable.
This seems to apply only to hardware Num lock toggling, as rapid toggling in onboard does not seem to cause the issue. The same does not appear to happen to the caps lock key. The state of num lock is indeed indeterminate when this occurs (e.g. numeric 4 will vary between typing '4' and left-arrow).
This appears to be directly related to prior CPU availability, as it is more of a problem on older hardware (e.g. with 2001 Pentium M) where it can be triggered on an idle system, and more difficult to duplicate on later hardware (e.g. with a dual-core processor) unless the CPUs are busy (e.g. other runaway processes).
Tested on both 32-bit and 64-bit. This behavior is present at least since Ubuntu MATE 14.10 (beta 1).
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ApportVersion: 2.14.7-0ubuntu1
Architecture: i386
DistroRelease: Ubuntu 14.10
InstallationDate: Installed on 2014-08-31 (1 days ago)
InstallationMedia: Ubuntu MATE 14.10 "Utopic Unicorn" - beta1 i386 (20140828)
Package: mate-settings-daemon 1.8.1-2
PackageArchitecture: all
ProcVersionSignature: Ubuntu 3.16.0-11.16-generic 3.16.1
Tags: utopic
Uname: Linux 3.16.0-11-generic i686
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo
_MarkForUpload: True
Possibly related to, if not duplicate of, bug 969359 (for gnome-settings-