I flew my Citabria from San Diego to Carefree AZ one Sunday. AMEX was down hard for most of the midwest terminals, they couldn't process any cards in a bunch of states.
So, I rushed to the datacenter, they waved me by security, I went in expecting to find an IBM 3745 in paroxis with flames coming out and nasty messages all over the screen. Hmmm - nope, all quiet here, except all the modem lines were in loop testing. Hmmmm - say that again. 'All - the - modem - lines - were - looping.' I walked over to rack, after rack, after rack of those black faced ATT modems and looked at the buttons on the front. Every one of them, the 'LPBK' button was in. So, I started to deselect them. Three rows of two racks per row with ~12 per rack. Checked the controller and no more loopback. Hopped in my plane and headed for my parents house in Prescott. That was an expensive mistake.