Physical Danger
Any time people are engaging in physical activity, there's the possibility of injury. Some individuals have been paralyzed following an accident on the bench press; others have slipped on a treadmill and banged their head on one of the handlebars on the way down. However, as your clients will be doing most of the work, they're in danger's way more than you are.
Your most serious risk may be that of a client lashing out and punching you in the chin when you're yelling at them to do just three more sit-ups. When you start seeing red in their eyes, you may want to back off a little.