The first one is chasing soreness as a sign of a good workout. Soreness mostly reflects unfamiliar movement, not effectiveness — and chasing it tends to push beginners into overtraining before their joints and connective tissue have caught up to their muscles.
The second is changing the program every week based on whatever shows up on social media. Adaptation takes consistency over weeks, not days — a mediocre plan followed for two months beats a great plan abandoned after four days.
Third: skipping the boring stuff — warm-ups, technique work on lighter loads, mobility — to get straight to heavier lifting. Most early injuries trace back to this, not to the heavy lifting itself.
Fourth: ignoring sleep and nutrition while obsessing over the workout itself. Training is the stimulus; recovery and fuel are what actually let it stick.
Fifth: training alone with no outside feedback on form. This is the single biggest reason coached beginners progress faster and safer than self-taught ones — someone catching a technique issue in week two instead of month six.