When using the Exterior Angle Theorem, it's important to avoid some common mistakes. Here are the key points to remember:

1. Understanding the Theorem: The Exterior Angle Theorem says that an exterior angle is equal to the sum of the two opposite interior angles. If you forget this rule, you might get the wrong answer.

2. Labeling Angles and Sides: Make sure to label your angles and sides clearly. It can be easy to mix up which angle is exterior and which are interior, especially in tricky diagrams. A simple labeling mistake can lead to wrong calculations.

3. Remembering Triangle Properties: Triangles have special rules. For example, the sum of the interior angles equals $180^\circ$. If you forget this, you might not fully understand how everything works together.

4. Knowing It’s For Triangles Only: The theorem only applies to triangles! If you try to use it for shapes that aren’t triangles, you’ll likely make a mistake.

5. Taking Your Time with Calculations: Don’t rush your math. It's easy to mess up if you're in a hurry. Take a moment to check your work.

Keep these tips in mind, and you'll do great with the Exterior Angle Theorem!