(Disclaimer: not all of these I agree with.)

• Being sedentary (sitting for hours without moving)
• Not warming up before exercise
• Not doing any muscle-strengthening activities
• Not taking rest days
• Skipping gym days
• Not cooling down/stretching after exercise
• Resting too long between sets
• Not varying cardio
• Taking dangerous stimulants
• Using bad form
• Rewarding exercise with junk food
• Not drinking enough water while exercising
• Working out on an empty stomach
• Lifting excessive weights
• Improper stretching
• Overtraining
• Phoning it in
• Taking steroids