Surface area and volume — cuboid, prism, cylinder, sphere, pyramid, cone

Finding the volume by mistake, not halving the triangle area, forgetting the slant face, or forgetting one triangular end.

Forgetting to divide the sphere formula by 2 for a hemisphere, or using $r^2$ instead of $r^3$.

Not knowing $1000\ \text{cm}^3 = 1$ litre (and $1\ \text{m}^3 = 1000$ litres); dividing by 100 or 10000.

For the curved surface use the slant height $l$ in $\pi r l$, never the vertical height. Add the base circle $\pi r^2$ only when the cone is solid or closed.

The surface is the two ends plus the curved part: $2\pi r^2 + 2\pi r h$. Dropping one circle (writing $\pi r^2 + 2\pi r h$) or leaving off the curved part is a common slip.