Lifters typically make the same two mistakes when it comes to nutrition, and both are related. But let's zoom out first and look at the big picture. First, we have to look at what most fat-loss diets have in common.
Diets make you stop eating (or greatly reduce eating) what we'll call "obvious crap" – the junk food, the treats, etc.
You know, the stuff that everyone knows isn't doing them any good, and the stuff they only seem to stop stuffing down their gullets when they're "on a diet." The result of ditching the junk? Overall food quality increases along with a natural reduction in calories.
In this sense, all diets work whether they're paleo, vegan, low-carb, low-fat, etc. A meat n' veggies paleo eater gets rid of the junk and so does the smug raw-food vegan. A non-celiac who drops the donuts and Lucky Charms gets results too, whether gluten is really an issue or not. This commonality tells us something: If you stop eating all the obvious bad shit, good things happen.
All diets make you pay attention.
Whether you're controlling your macronutrients or meeting a certain calorie count, you have to snap out of it and be aware. You go from a mindless eater to a mindful one. You're forced to read a label or two, which always works wonders, regardless of the diet's methodology.
This is why the simple advice of "keep a food log" usually leads to better nutrition and fat loss, even if no guidelines are given to the dieter – he or she is now aware and thinking about nutrition.
So that leaves us with two things that always work with any eating plan: Stop eating obvious garbage and start paying attention to what you do eat. And that circles back around to the biggest mistakes lifters make with diet: they won't stop eating obvious garbage, and they are unaware, mindless eaters.
Well, not many people want to hear it, but here goes: Stop stuffing garbage into your body and pay attention. Forever. Whether you're "on a diet" or not.
Yes, you'll suffer from very real withdrawal symptoms at first and it will suck like a Dyson, but then those feelings will fade, healthy food will start to taste awesome, your mind and body will self-regulate naturally, and you'll never have to really diet again.
Oh, and if you do decide to have a birthday treat, that one "bad" meal won't avalanche into six months of dietary debauchery and FUPA-gains. Auto-regulation and the elimination of food addictions has put you back in control.
Or you could adopt countless diets, fail again and again, be miserable, let food control you and, in the end, wreck you. And that stinks because food is amazing, learning how to prepare it is an invaluable skill, and striving to be healthy, strong, and lean is the grown-up thing to do.