Helped someone figure out a weird problem just now. He would telnet to a router and then telnet back out to a host. He would then try the ctrl-shift-6, x sequence and he would get nothing on the screen. He could type disconnect 1 and get no feedback but as soon as he pressed enter it would show the router prompt and immediately reconnect the session. Seems like it was only listening to the "enter".
Turns out it was an echo problem. In his Putty settings he had Terminal:Line discipline options set to Force On for both Local Echo and Local line editing. He set these both to Auto and it worked fine after that.
Not sure how that get set but I thought I'd share. A quick google didn't turn anything up that seemed similar.