Trigonometry — SOHCAHTOA (2D and 3D)

Choosing cosine when sine is needed (or vice versa) because the sides were labelled wrongly relative to the angle.

Dividing when you should multiply (or vice versa) when the unknown is in the denominator.

Working in gradians or radians instead of degrees gives wrong values.

Writing $\theta = \dfrac{\text{opp}}{\text{adj}}$ and giving that as the angle, instead of $\theta = \tan^{-1}\!\big(\dfrac{\text{opp}}{\text{adj}}\big)$. The inverse turns the ratio into the angle.

When the unknown is on the bottom, e.g. $\sin 35 = \dfrac{7}{\text{hyp}}$, multiplying by $\sin 35$ instead of dividing. Rearranged: $\text{hyp}=\dfrac{7}{\sin 35}$.