Can a diagnosed benign tumor be found to be cancerous after it's removed?

Yes. When one removes a tumor, it is examined by the pathologists under the microscope. In addition, special stains are performed on the tissue to assess whether the tumor might be malignant. So, in certain instances, something that was thought to be benign, may, in fact, be a malignant cancer.
Yes. Sometimes a tumor can be diagnosed as benign on an initial biopsy, but when the entire tumor is removed the pathologist may see an area that meets the criteria for a cancer.