1. Go too hard, too fast on a 48hr+ fast (pace yourself!)
Fasting is a marathon, not a sprint. If you’re new to extended fasting and you’re also doing a high-intensity work out at the same time, you’re going to have a bad time. Pace yourself, save the really intense gym days for days you’re doing shorter fasts or pick an exercise like walking, un-heated yoga, etc. for days that you are doing these longer fasts. Fasting already puts your body under some degree of stress (healthy stress, but still stress). Make sure you’re listening to what your body can handle during these periods!
2. Being scared to try exercising - don't be! Just listen to your body's needs
You can absolutely exercise while fasting. And the more you do intermittent fasting with exercise the easier it will get. Just work up gradually and really listen to what your body needs on a particular day. As a general rule, save the harder workouts for shorter fast days and more gentle exercises for extended fasts.
3. Not drinking enough water
Your body is going to need water to break down your fat to use it for energy. You will likely find yourself drinking a lot more water (especially as you become fat adapted) than you would have before. Be prepared for this! And, make sure you always have plenty of water with you for your workout and recovery.
4. Not taking electrolytes (particularly salt, magnesium, and potassium)
On extended fasts, you’re going to want to make sure you’re getting plenty of electrolytes, particularly Salt, Magnesium, and Potassium.
5. Telling people you're fasting (many still have outdated beliefs about fasting, best to keep it a secret)
As they say, the first rule of both fasting (and Fight Club) is you tell no one about Fasting Club. Especially when you’re new to this and still getting your fasting legs, be careful about who you share this with. The science on fasting is fantastic, but a lot of well-meaning people are not well-educated on it yet and may try to talk you out of fasting, because they don’t know better. It is best to wait to tell them you’re fasting until they see your unbelievable results and are asking about what you’re doing!