[stewartm]

Getting out of telnet? Mere bagatelle compared to trying to get out of `ed` for the first time. At least Telnet attempts to tell you what to do.

The defacto ed tutorial exactly predicts/describes my first encounter:

https://sanctum.geek.nz/arabesque/actually-using-ed/

[twic]

Let's look at a typical novice's session with the mighty ed:

  golem$ ed
  
  ?
  help
  ?
  ?
  ?
  quit
  ?
  exit
  ?
  bye
  ?
  hello?
  ?
  eat flaming death
  ?
  ^C
  ?
  ^C
  ?
  ^D
  ?

Note the consistent user interface and error reportage. Ed is generous enough to flag errors, yet prudent enough not to overwhelm the novice with verbosity.

-- https://www.gnu.org/fun/jokes/ed-msg.en.html

[stewartm]

:) Had not spotted the humour section on gnu.org before.

From the same site, the source code of ed:

while :;do read x;echo \?;done

-- https://www.gnu.org/fun/jokes/ed.html

[JdeBP]

Having already learned that EOT (Control+D) exited the shell, I found exiting ed and cat fairly easy.