Saved By the Bell...

Meaning: "Saved by a last minute intervention."

Origin
This is boxing slang that came into being in the latter half of the 19th century. A boxer who is in danger of losing a bout can be 'saved' from defeat by the bell that marks the end of a round. The earliest reference to this that I can find is in the Massachusetts newspaper The Fitchburg Daily Sentinel, February 1893:

"Martin Flaherty defeated Bobby Burns in 32 rounds by a complete knockout. Half a dozen times Flaherty was saved by the bell in the earlier rounds."

There is a widespread notion that the phrase is from the 17th century and that it describes people being saved from being buried alive by using a coffin with a bell attached. The idea being that, if they were buried but later revived, they could ring the bell and be saved from an unpleasant death. The idea is certainly plausible as the fear of burial alive was and is real.