When something really good happens unexpectedly. The bonus sometimes has a double meaning, as it often can be viewed as something negative or a tragedy. This is not a requirement of a double bonus however, as 'double' does not mean the bonus must have double meanings, it means the bonus is really good. often only because it is so unexpected.
Person 1: "So this cop was chasing me on the freeway and he wrecked..."
Person 2: "Nice. Double bonus."
Person 1: "I went into our new roommates room to turn off his alarm and I found a naked stripper tied to his bed."
Person 2: "Ah, but would you call that a bad thing, or double bonus?"