#1 Fat Loss Mistake
There are so many potential answer to this question based on who you ask, but to us, the #1 fat loss mistake we see people making has to be slashing calories too fast and eating too little. Fat loss is an emotional process and it is easy to abandon reason for irrational fears.
“I'm not losing weight, I need to cut calories by another 500 per day!”
Fat loss takes longer than any of us ever hoped. But make no mistake, slashing calories to try and kick start fat loss will actually produce the opposite effect. In the first place, your body can only lose so much fat per week. If your caloric deficit is larger than the amount of fat your body can lose during the same time period...your body will start sacrificing muscle. And this is really bad. Your muscle is what keeps your strength up, your training sessions productive, and your calories expended high.
Secondly if you reduce your calories by too much, too soon, your energy will tank. You'll become a zombie in the gym. Weights will feel heavier, it'll be harder to focus and having productive workouts will turn into a struggle. The caloric amount you eat largely influences the amount of energy you can apply at the gym.
These kinds of negative effects are bad in themselves, but will also affect your mental state and make it harder and harder to stick to your diet. If you see the weights going down in the gym, and you feel more and more wiped out and tired, you are way more likely to cave in to cravings.
So take a long term view, set yourself up for success and try to eat as much as you possibly can while still losing weight.