If your S-Video cable between a satellite or cable box and the TiVo goes bad, and you try the yellow composite video RCA jack instead, the TiVo doesn't know to look at that jack for incoming video, it's still looking on the S-Video jack until you re-do setup.
And yes, S-Video cables can go bad just sitting there, as unlikely as it seems to you and seemed to me until it happened.
"I am altering the deal. Pray I don't alter it any further."
Darth TiVo, 14 February, 2011