The adhesive covering the back sensor is loose or is letting in light. Reapply the adhesive.
The cutting project is beyond allowable margins. Move the project to within limits or use a larger mat.
Project or page size is bigger than the paper size. Check the Panels box on the Cutting Control Panel. If you see 101, 202 etc. your project is larger than 12x12. If you are cutting paper bigger than 12x12, change the material size in the Cutting Control Panel (for example, for 15" paper, change the material to 15.000)
You were cutting a larger page, switched to a smaller paper, and forgot to move the wheels to fit the smaller page. If this is the case, move the wheels. The silver rod that helps to grip and move the paper might b