A completely fake reboot

zsh + coreutils + unclutter + amixer + xterm (Arch Linux)

I took the answer by @TheDoctor and ran with it. This version has many improvements, and is 99% convincing to an experienced user (me) on my Arch Linux system. I use Zsh since it has good array and floating-point number support.
Dependencies: feh, unclutter, amixer, zsh, xterm

Improvements:

1) Use the number printed in the first column by dmesg, which is the time since boot, (e.g. [ 0.000000] ) to determine the time to sleep. Without this it looks very unrealistic on my machine. These times are parsed before the loop (in an early call to sleep) since parsing inside the loop is too slow.

2) Don't print lines where time since boot is larger than 16 seconds. This specific number is machine-dependent, but the point is to avoid printing later dmesg-stuff that comes from inserting/removing usb sticks, etc. and is unrelated to booting.

3) Do all this in a fullscreen terminal window with black background and white text. Kudos to Mechanical Snail for this trick used in: Make a PNG image with "Hello World!" with programming APIs, in the shortest code possible

4) Mute the audio on shutdown, restore volume when script finishes.

5) Hide the mouse cursor, restore when script finishes.

6) Show BIOS and Syslinux splash screens.

Run with: xterm -fu -fg white -bg black -e '/usr/bin/zsh fake-reboot.sh'

Code:

#!/usr/bin/zsh
# Remove (undisplay) the mouse pointer
unclutter -idle 0 -jitter 255 &
# Since there is no easily-accessible (i.e. without being root) shutdown log, we
# fake these messages.
echo "The system is going down for maintenance NOW."
sleep 2.0
echo "[21656.404742] systemd[1]: Shutting down."
echo "[21656.404742] systemd[1]: Stopping Session 1 of user `id -u -n`."
echo "[21656.404742] systemd[1]: Stopped Session 1 of user `id -u -n`."
echo "[21656.404742] systemd[1]: Stopping Sound Card."
# For added effect, store volume and then mute sound
volume=`amixer -- sget Master | awk -F'[][]' 'END{print $2}'`
amixer -- sset Master 0% &> /dev/null
echo "[21656.404742] systemd[1]: Stopped target Sound Card."
sleep 0.5
echo "[21656.919792] systemd[1]: Stopping system-systemd\x2dfsck.slice."
echo "[21656.919792] systemd[1]: Removed slice system-systemd\x2dfsck.slice."
echo "[21656.919792] systemd[1]: Stopping system-netctl\x2difplugd.slice."
echo "[21656.919793] systemd[1]: Removed slice system-netctl\x2difplugd.slice."
echo "[21656.919793] systemd[1]: Stopping User Manager for UID `id -u`..."
sleep 0.7
echo "[21657.624741] systemd[1]: Stopping Graphical Interface."
echo "[21657.624742] systemd[1]: Stopped target Graphical Interface."
echo "[21657.624745] systemd[1]: Stopping Multi-User System."
sleep 0.9
echo "[21658.606561] systemd[1]: Stopped target Multi-User System."
echo "[21658.606562] systemd[1]: Stopping Paths."
echo "[21658.606562] systemd[1]: Stopped D-Bus System Message Bus."
echo "[21658.606562] systemd[1]: Stopped target Paths."
echo "[21658.606568] systemd[1]: Stopping Timers."
echo "[21658.606568] systemd[1]: Stopped target Timers."
echo "[21658.606568] systemd[1]: Stopping Sockets."
echo "[21658.606568] systemd[1]: Stopped target Sockets."
echo "[21658.606568] systemd[1]: Starting Shutdown."
echo "[21658.606571] systemd[1]: Reached target Shutdown."
echo "[21658.606571] systemd[1]: Starting Exit the Session..."
echo "[21658.606571] systemd[1]: Received SIGRTMIN+26 from PID 10988 (kill)."
echo "[21658.606571] systemd[1]: Deactivated swap."
sleep 0.4
echo "[21659.001741] systemd[1]: Starting Unmount All Filesystems."
echo "[21659.001742] systemd[1]: Unmounted /home."
echo "[21659.001742] systemd[1]: Reached target Unmount All Filesystems."
echo "[21659.001742] systemd[1]: Stopping Remount Root and Kernel File Systems..."
echo "[21659.001742] systemd[1]: Stopped Remount Root and Kernel File Systems."
echo "[21659.001743] systemd[1]: Reached target Shutdown."
echo "[21659.001743] systemd[1]: Starting Final Step."
echo "[21659.001743] systemd[1]: Reached target Final Step."
echo "[21659.001754] systemd[1]: Shutting down."
sleep 0.3
echo "[21659.304341] systemd-journal[250]: Journal stopped"
sleep 0.2
echo "System halted."
sleep 2
clear
sleep 1
# Get the BIOS splash screen and display it
wget http://pvv.ntnu.no/~asmunder/bios.jpg  &> /dev/null
feh -Z -x -F -N --force-aliasing bios.jpg &
pid=$! # Store PID of Feh, so we can kill it later
# While showing the BIOS splash, use the time to parse dmesg output into arrays
tim=`dmesg | awk '{print $2}' | sed 's/]//' | grep "[0-9][0-9][0-9][0-9][0-9]"`
tim=($=tim)
dmsg=("${(@f)$(dmesg)}")
sleep 2.5
kill $pid
sleep 0.5
# Get the Syslinux splash and display it
wget http://pvv.ntnu.no/~asmunder/syslinux.png  &> /dev/null
feh -Z -x -F -N --force-aliasing syslinux.png &
pid=$!
sleep 1.3
kill $pid
# Loop through the arrays we created. Calculate the time we have to wait before
# displaying this line. If the wait time is less than 0.1 sec, we skip waiting.
T1=0.0
T2=0.0
n=0
for d in $dmsg; do
  T1=$T2
  T2=${tim[$n]}
  ((dT = $T2-$T1))
  if (( $dT > 0.1));then
    sleep $dT
  fi
  echo $d
  if (( $T2 > 16.0 )); then
    break
  fi
  ((n=$n+1))
done
sleep 1
clear
# It's normally agetty that parses /etc/issue and handles escape codes in a 
# special way. Thus we skip the first line of /etc/issue and do that manually.
echo "Arch Linux "`uname -r`" (tty1)"
tail -n +2 /etc/issue 
echo `hostname`" login:"
sleep 10
# Reset the mouse pointer so it is visible again
unclutter -idle 5 -jitter 0 &
# Reset the audio volume
amixer -- sset Master $volume &> /dev/null

Commodore 64

1?CHR$(147)
2?"    **** COMMODORE 64 BASIC V2 ****"
3?
4?" 64K RAM SYSTEM  38911 BASIC BYTES FREE"

The BASIC intepreter will display the READY. prompt automatically.

Bash + Coreutils (Linux)

echo "The system is going down for maintenance NOW."
clear
sleep 5
dmesg|while read i; do echo "$i"; sleep 0.1; done
cat /etc/issue
login