The micrograph shows a section of cardiac muscle with a piece of thyroid tissue against one surface. It is obvious that one piece of tissue must be a contaminant and in this instance it is the thyroid tissue. It is possible however, for undetected contamination of a specimen to occur with tissue of a similar type (for example endometrial tissue with endometrial fragments). This in turn could lead to an inaccurate diagnosis which could have serious consequences for a patient. It is therefore important to remove the potential for the occurrence of this artefact as much as possible. So the answer is cross contamination specimen to specimen.