Comment 3 for bug 1539546

Still visible today during an upgrade from 15.10 to 16.04:

Setting up lvm2 (2.02.133-1ubuntu9) ...
Installing new version of config file /etc/init.d/lvm2 ...
Installing new version of config file /etc/lvm/lvm.conf ...
update-initramfs: deferring update (trigger activated)
Processing triggers for ureadahead (0.100.0-19) ...
Processing triggers for libc-bin (2.23-0ubuntu3) ...
Processing triggers for systemd (229-4ubuntu4) ...
Processing triggers for initramfs-tools (0.122ubuntu8) ...
update-initramfs: Generating /boot/initrd.img-4.4.0-18-lowlatency
W: Possible missing firmware /lib/firmware/ast_dp501_fw.bin for module ast
W: plymouth: The plugin label.so is missing, the selected theme might not work as expected.
W: plymouth: You might want to install the plymouth-themes and plymouth-label package to fix this.