Suppose.... Your opponent barely nudges the rack on a break shot. With no good shot after the break, he skims the side of the 14-ball rack and nearly freezes the cue ball to the end cushion. In skimming the rack, he has moved a ball so that it looks like there is a combination out of the rack. The safe is very difficult to play. If you choose to play safe, you have much less than a 50% chance to get the first open shot. Oh, and you're already on two fouls.
The combination out of the rack looks close, and you estimate it to be a 45% chance. There is no other problem with the shot: it can't be kissed out and the balls will be well-broken and the cue ball is very unlikely to scratch.
In this situation, you should shoot the combination even though it is less than 50%.