6.2.2.3. Commands that switch the editor to insert mode
Pressing the Esc key switches back to
command mode. If you're not sure what mode you're in because you
use a really old version of vi that doesn't
display an "INSERT" message, type
Esc and you'll be sure to return to command
mode. It is possible that the system gives a little alert when you
are already in command mode when hitting Esc,
by beeping or giving a visual bell (a flash on the screen). This is
normal behavior.