I'm using fish, but I've tried doing
konsole -e bash as well (it inserts a visible "s") there. It seems pressing certain Ctrl-* key combos inserts a Ctrl-S before actually sending that particular escape sequence in. For instance, Ctrl-L clears the screen and inserts an @s after my prompt, as if I'd typed it in before the Ctrl-L.
I've disabled the "flow control" thing already.