FWIW, this just happened to me too, on 16.04.2. It does this once every
few weeks, trigger unknown. Fixed by toggling repeat setting off/on.
Guess I would rate this as a minor annoyance only.
I also have Nvidia so this could be the common factor:
ECS$ lspci | grep -i nvidia
01:00.0 VGA compatible
sudo purge-old-kernels exists for me (Ubuntu 15.04) but fails because
... /boot is full :p
[snip]
update-initramfs: Generating /boot/initrd.img-3.19.0-50-generic
gzip: stdout: No space left on device
E: mkinitramfs failure cpio 141 gzip 1
[snip]
This is such a long-standing issue, have to wonder