Pythagoras' theorem (2D and 3D)

When finding a shorter side, students add $a^2+b^2$ instead of subtracting, scoring zero for the method.

Rounding an intermediate length (e.g. 6.7 → 7) then using it loses the final accuracy mark.

Stopping at $a^2+b^2$ (the squared value) and quoting that as the length. You must take the square root: e.g. $\sqrt{36+64}=\sqrt{100}=10$, not $100$.

Applying $a^2+b^2=c^2$ when no right angle is marked. Pythagoras only holds for a right-angled triangle, so check the diagram shows a right angle first.

For a space diagonal, using only two dimensions — $\sqrt{l^2+w^2}$ — instead of all three: $\sqrt{l^2+w^2+h^2}$. Find the base diagonal, then combine it with the height.