This mistake is probably the most common one which allows other muscle groups to "steal" tension from your biceps and allowing your biceps to not work as hard as it should be. Here are some observations to know if you are making this mistake:
Observation 1: Shrugging as you try your best to curl up the weight - Guess the "tension thief" muscle group!
If you guessed the TRAPS, you are absolutely right! If you spot this observation when you're curling next time, the weight is probably too heavy for you as your body is naturally using your traps to curl up the dumbbell for you.
Solution: Try pushing your shoulders down to try to create the maximum distance between your shoulders and your ears before curling. This would prevent your trapezius muscle from affecting the movement and tension on your biceps
Observation 2: - Elbows moving forward as you curl the weight up. Guess the "tension thief" muscle group!
When you're doing some dumbbell curls next time, try facing sideways to spot this mistake. If you observe your elbows shifting back and forth, this means that you are using your front deltoids to take away tension from the biceps.
Solution: Try lowering the weight and ensuring the perfect form by making sure that your elbows are held in a fixed position. If you find difficulty in doing so, try leaning back against a wall and making sure your elbows are constantly "stuck" to the wall as you curl. This would definitely make sure that your front deltoids are not activated and only activating your biceps!
Observation 3: - Bending your wrists as you curl the dumbbell all the way up. Guess the "tension thief" muscle group!
Another subtle mistake is activating your forearms as you bend your wrists during the curl to flex your forearms. This not only takes tension away from the biceps but puts unnecessary stress on your wrists.
Solution: Needless to say for this one, keep your wrists in a neutral position or if you have to, put on some wrists straps to permanently keep your wrists straightened to prevent the forearms from taking away tension from your biceps.