Good day!
A friend of mine just pick this bug (whatever).
1. run telnet
2. on telnet> prompt do:
- press and hold 'Ctrl' button
- then press '~' button
- then press '\' button
- unpress the 'Ctrl' button
3. You'll see this:
> telnet> ^\Quit (core dumped)
It plays in all tested 6.x/7.x with tcsh/zsh/xterm.