There's no mercy rule in the NFL. The next closest thing, though, might be the indignity of the national broadcast waving the white flag. That's what Fox did with Sunday's 49ers-Bucs broadcast when things got out of reach for Tom Brady's team.
Most markets, except for the Carolinas and the Pacific Northwest, were treated to the Niners-Bucs stinker in Fox's late afternoon slot. Outside of the Bay, most of the country had the plug pulled on the game extremely early. (Local markets get their team's games all the way through, unfortunately for any masochist watching the second half in the Bay Area.