Sine rule, cosine rule, area = ½·ab·sinC

Even though both are on the formula sheet, students copy them down wrongly.

Algebra slips when solving $\tfrac12 ab\sin C$ for a side, or the cosine rule for an angle.

Using a non-included angle (not between $a$ and $b$) in $\tfrac12 ab\sin C$.

Computing $\cos A = \dfrac{b^2+c^2-a^2}{2bc}$ but giving that decimal as the angle. Take the inverse: $A=\cos^{-1}$ of it, e.g. $\cos^{-1}(0.2)=78.5°$.

Using the sine rule for two sides and the included angle (SAS), where only the cosine rule works, or the cosine rule when a side and its opposite angle are known. Match the rule to the data first.