bug in telnet client?

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.