Final answer:
Using a pen to label the origin of the TLC plate can lead to inaccurate or irreproducible results due to the interference of ink substances.
Step-by-step explanation:
Using a pen to label the origin of the TLC plate can lead to inaccurate or irreproducible TLC results. The ink of the pen may contain substances that can interfere with the separation of the compounds on the plate. This can lead to distortion of the spots and inaccurate measurement of the distances traveled by the compounds.