When configuring a cisco router or switch, you may have been told that you can stop your console or telnet sessions from timing out by using the following configuration:
line con 0 !(or line vty 0 4) no exec-timeout !DON'T DO THIS!
NEVER issue the command “no exec-timeout” – instead it is much safer to use “exec-timeout 0”, because people make mistakes when typing “no exec-timeout” and end up with “no exec”, which means “do not run the Cisco IOS exec under this setting”. It can be used as a way of totally disabling the AUX port, (under line aux 0) or even disabling telnet (under line vty 0 4)
line con 0 !(or line vty 0 4) exec-timeout 0 !Much safer
RedNectar's Blog 
What a hell of a tip! I was at my wits end wondering why the routers I just bought on eBay were inaccessible after the router timed out even though I was using, or so I thought using correctly, the “no exec-timeout” command. I was about to send them back when I did a quick search and found your page.
Thanks a bunch for saving me return shipping money.
Glad it helped
Thanks for the tip! I just saw a MicroNugget from Keith Barker on how to improve GNS by applying Idle PC values and the no exec-timeout, then googled around and came to your post, thanks for explaining it!
I knew it! i made this mistake a few minutes ago with my 2611xm, left my desk to eat something and when back the console session timed out and totally lost console access. An horrible experiencie, indeed horrible.
How did you recover from device?
I had to essentially do a password recovery – except instead of replacing the password, I had to remove the “no exec” like this:
Thank you so much for the explanation of no exec-timeout command. It was disturbing me. Justin
No Problem. Glad it helped.