Mistake #1: Writing if (x == true) instead of if (x). If x is already boolean, the comparison is redundant.
Mistake #2: Confusing = (assignment) with == (comparison). Writing if (x = 5) assigns 5 to x and evaluates the result, not what you wanted.
Mistake #3: Forgetting operator precedence. !a && b groups as (!a) && b, not !(a && b). Use parentheses to clarify intent.