Writing and erasing are two of the thirty-nine melachos of Shabbos that were performed in the building of the mishkan. Each board used in constructing the mishkan was marked so that it would be returned to its correct place when the mishkan was reassembled (Rashi Shabbos 73a; Gemara Shabbos 103b). (The Talmud Yerushalmi emphasizes the importance that each board be kept in the same place [Shabbos 12:3].) The numbers written on the boards were also sometimes erased if someone made a mistake.