Common mistakes to avoid

Adding or subtracting decimals without lining up decimal points.
Treating subtraction as commutative.
Treating division as commutative.
Dividing by zero.
Forgetting to work left to right for multiplication/division or addition/subtraction.
Adding fractions before finding a common denominator.
Multiplying fractions by adding numerators and denominators.
Dividing fractions without using the reciprocal of the divisor.
Dropping negative signs during multi-step calculations.
Confusing percent form with decimal form.
Using $25$ instead of $0.25$ for $25\%$ in calculations.
Moving the decimal in only one number when dividing decimals.
Rounding too early in a multi-step problem.
Forgetting that $1$ is neither prime nor composite.
Assuming an estimate must equal the exact answer.

Sources

OpenStax Mathematics
Mathematics LibreTexts
Stewart, Calculus: Early Transcendentals
Lay, Linear Algebra and Its Applications
Rosen, Discrete Mathematics and Its Applications
Boyce and DiPrima, Elementary Differential Equations and Boundary Value Problems
Blitzstein and Hwang, Introduction to Probability
Parell GitHub repository