Combination shots come up a lot in straight pool and 9-ball. The most common combination shot in 9-ball is of course targeting the 9-ball. Setting yourself up for a 2-9 or 3-9 combo is very common. Over and over again I notice players missing basic combos. This is why it is a great idea to start out with a very basic combination shot and work your way foward onto harder combos and more difficult positioning of the cue-ball after the combination shot is successful.
This may look easy but concentration is very important.