The ball hitting things in the classroom (and even breaking them)
Students getting overexcited
The ball distracting the students from the language
Students who can’t catch (well) getting frustrated or holding the game up
Students fighting over who should have the ball next, who won a point, etc
Time wasting, e.g. by students throwing the ball far away from the next person
Students doing nothing while waiting (maybe a long time) for the ball
Students being picked on (by not having the ball thrown to them, having the ball thrown to them too often or too hard, having the ball thrown to a place that is difficult to catch, etc)
Students getting an uneven amount of time speaking (due to how often they are thrown the ball, how long they can spend with the ball depending on how successful they are, etc)
Students getting bored with the same old uses of the ball
The many variations mentioned and then summarised below have been designed to deal with all of those problems, as well as to make using a ball possible for a wider range of language points, more fun and more productive.