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When switching between terminals and text editors the occasional ctrl-s gets accidentally typed into the terminal. For along time I thought that this was a bug in screen but it is a standard terminal feature. Ctrl-s calls the software flow control method XOFF which stops the character flow to the terminal, which when you did not realise what you pressed just seems to freeze the terminal.

ctrl-q calls XON and starts the terminal again.

For a more permanent fix you can add this to your .bashrc (not sure how other shells are effected).

stty ixany

Which allows any character to call XON, so the character press is sent and displayed and you will never be aware of the terminal freezing.

If this does not work for you, or you also don't seem to be able to send ctrl-s ctrl-q commands to terminal applications you can use
stty stop undef To unmap ctrl-s
stty start undef To unmap ctrl-q

Or to stop XOFF and XON being sent from the keyboard but still allowing other software to send the commands use
stty ixoff -ixon
NB: ctrl-q can now be used to shutdown rtorrent