When just starting to work out what is the worst mistake that can be made?

Heavy weights and improper form were used.For the first 1–3 months, no heavy weights are needed.Light weights will be used for more gains.Tell that to a teenager at Planet Fitness.

Forgetting how to breathe.When you do exercises, you should follow breathing tips.Unless your fat is properly burnt.If you forget breathing tips, it may be the worst mistake.Without proper breathing practice, working out is useless.