Harry Potter has got a knife that can open any locked door from his godfather Sirius Black. This knife gets destroyed in Order of the Phoenix chapter 34.
When did Sirius acquire the knife? Presumably he didn't have it with him in Azkaban, because it's unlikely that they let him keep such an item in prison. Did he already have that knife before he was taken to prison, but has hidden it somewhere outside, such as in the Black Manor? Did he have it when he broke into the Gryffindor tower in Prisoner of Azkaban chapter 13, or before that, when he slashed the Fat Lady's portrait?
Also, how did Sirius acquire his knife? Is this an ordinary item you can just buy in a shop (if you're not a fugitive searched by the Ministry of Magic that is)? Is it rare and valuable? Do we know its story?