daf bit: Sanhedrin 48

Items set aside for holy purposes cannot then be used for ordinary ones; you cannot use the bag that holds your t'fillin to carry money, for instance. What does "set aside" mean -- using it for a holy purpose, or designating it explicitly? On today's daf the g'mara discusses tombs. If a man builds a tomb for his father and then buries his father elsewhere, he cannot use the tomb for himself. Is this because of designation (meaning that designation was sufficient)? The g'mara answers that this is not a case of designation (so it cannot help to settle that question) but, rather, you can't use your father's tomb out of respect for your father. (48a)

(Some themes come up in multiple places in talmud. I found this designation-versus-usage question here interesting because my rabbi and I were just studying this discussion in a completely different tractate (B'rachot) last week. It wasn't resolved there either.)