Can there be mistakes in grading cancer?

Possible: Grading of cancer is done by pathologists based on the microscopic characteristics of the tumor. It is partly a subjective process and two pathologist may grade a tumor differently, you could call that mistake.

Usually NO: Pathologists are the one who grade the cancer based on specific parameters and criteria.. They are highly trained in this, if any dout they consult with other experts before releasing the final report and rarely wrong.

Grading: There is always some subjectivity in grading cancers. Pathologists evaluate the way cells look under the microscope to make the determination on grading. Many pathology departments will do an intra departmental peer review or second opinion, especially for diseases such as prostate cancer where grading makes a difference how a patient is treated.