1. The cue ball is pocketed
2. The cue ball is popped off-table
3. The cue ball (or object ball) fails to touch a rail after contacting another ball
In a more common context, the word "scratch" is applied to the activity of rasping, or digging with the fingers, or nails. Examples of this useage can be seen in the phrase, "You scratch my back, I'll scratch yours." In this slang context, that phrase is amended to, "You suck my junk, I'll lick yours." Of course, those verbs can be further amended to allow for gay/lesbian scratching, or different oral techniques.
"Golly, Jane! That'd be swell!"
"Sure, Sue, I think I'll just scratch the snatch for a bit... take your time in there."